To help you get the firmest possible grasp of the principles that underlie the approach I advocate, I thought it might be useful to make a few notes.
To start, the acquisition of flexibility by adults is a completely different proposition than with children, the group you usually work with. Adults, by definition, have experienced their second growth spurts, usually (but not always) in their late teens. There are many reasons for this critically important difference, and these can be canvassed below if anyone's interested. The key point here is that standard methods (like "hold a stretch for 30") will not be effective in changing any present patterns that adults have. Much experience has shown this to be accurate.
This reality suggests that a different approach is required, and that is what I want to address here. I can say that I have tried every approach that has been written about, and many that have not. What I want to share with you here has never been written down, apart from oblique references in my past books. What follows are the core conditions for an adult to change his/her body–mind in a way that observers would describe as "he/she has become more flexible". I can expand on this aspect, but what is not obvious to anybody is that one's pattern of flexibility is actually one's "self": one's personality, self-beliefs, fears, and so on. One's emotional self is precisely this pattern.
Recall the immediate changes that you saw on the workshop where you first saw this method in action: the young man's appearance, projection of personality, confidence, and many other aspects were different the moment he came out of the stick stretch. The basic person did not change, but the presentation of the person did. You are interested in flexibility insofar as it applies to better execution of skill elements in your sport; but many other benefits will accrue in the pursuit of this goal, and all will be regarded as a benefit to you and to the individual. When we talk of body language, this complex patterning is what we refer to.
The essential conditions for flexibility to change have two parts. One is the exploration of new ranges of movement, and the other is how this can be 'embodied' (retained in the body and incorporated in the activity in question).
The #1 necessity is that, when stretching, heat must be kept in the body: the work of remodelling fascia cannot be done in shorts or bare legs. In time, once that new flexibility has been embodied, they will be able to demonstrate it in any kind of clothes—but if you want to remodel their present flexibility, heat has to be retained.
A side note: whiles additional contractions can be performed, and new ROM explored, we are working on the somatosensory cortex and what tension it believes is necessary or useful. We are remapping what the unconscious part of the brain believes is the appropriate length-tension relationship in the various body parts. But when no more improvement in ROM can be achieved, we are now up against restrictions in the fascial structures themselves. Maintaining as much heat as possible in the body allows gentle and slow fascial remodelling. The way this is done is to back off slightly from the maximum ROM end position, and wait—minutes, for some muscle groups (this requirement depends on relative muscle size). More on this below.
Maintaining heat can only be done by using dual layers of tights and thick cotton tracksuit pants in my experience. This is why dancers wear leggings (there is a fashion element there, too!). There will be no problem in learning to see the body's alignment through track pants and the upper body can be naked or lightly clothed: keeping the heat in the largest muscles that is the key requirement. Ambient heat is no help here: the human body is expert at shedding heat (the result is that no matter what exercise is being done, or what the ambient temperature is, the human body core temperature hovers around 98.6 F, unless something goes wrong, like rhabdomyolysis).
Only a very narrow window of increased temperature in the muscles is required to open the window to changing one's patterns (2 degrees Celcius). To put this in perspective, a lukewarm bath is 40 degrees C (a fraction above body temperature) and a scalding hot bath that you could not immerse yourself in is only 44 degrees. The point is that the reactions that we are trying to influence in the body change radically over very small temperature variations. This 'window' can be opened by slowing the body's normally very effective temperature shedding strategies by wearing the recommended gear, and worn on the bottom half of the body only.
So, tights and track pants, please, and learn to read alignment through these clothes. It's easy to see, but practise is required.
The second and equally important point is that flexibility cannot be achieved by force or by intensity. I know this is counterintuitive to a degree, because we have to exert some force to provoke any change (in strength training or in any other) and in flexibility work, effort is needed. The contractions we recommend require force. But, and this is a huge but, the force is used only to make, or re-make, the connection to that part of the body. Once the force has been applied, the body has to be brought to a state where it's willing to let this protective tension go. Recall our conversation re. anaesthesia: all humans have perfect flexibility while under anaesthetic; as they regain consciousness, though, individual patterns re-manifest. The point is that force cannot change the pattern: the trigger to change this is not consciously available to us.
In fact, in the ST system, we use the bones, muscles, and fascia only to remap the brain; this is what provokes the changes we regard as "becoming more flexible" in the short term. Further, heat allows the fascia to be remodelled once the elongation is experienced. Both are necessary. Effort is only required to the extent needed to provide the proprioceptive feedback to that part of the brain that decides how much tension to maintain in any body part, and its pattern around the body. As well, the degree of force that is required to bring this change about cannot be known ahead of time. Personally, now, I need 80–100% contraction force; other students need only 10%, and any level above 50% in these students actually has the opposite effect (the body experiences the force in the stretch as a direct threat, and literally creates additional tension to ensure the elongation does not happen). The capacity to tolerate more tension (hence stronger intensity of the stretching experience) can be learned; but it cannot be imposed: it has to be allowed, and can only be experienced, and learned.
To achieve this goal of knowing 'how much', each individual's attention has to be turned inwards. No teacher can do this part of the process. This redirection of attention will be a complete change in approach (because the way of working in most group facilities is that athletes' attentions is on the coach and on your reactions/suggestions following the practising of the activity). But what I am recommending here only needs to be done twice a week at the end of the hardest session (strength-wise) to be effective. The change of attention (from the leader of the activity to each athlete's internal state) will not affect all the other psychological aspects of their training.
The larger point is this: unless the athlete's brain and sensory being is directly involved in the experience of stretching, it will not be effective. So, just for this part of the training week, you will need to ask your athletes the most important questions for the acquisition of flexibility: What does that feel like, and where do you feel it, and how can you relax further into it?
The deeper point here is that only each individual can answer that question and—critically—the time it takes to relax into the beginning of a stretch; the contraction time, and the time spent in the re-stretch is unique. It can only be experienced, then learned, by each of the athletes themselves; it cannot be reduced to a formula of number of seconds, or number of reps, or percentage of maximum strength in the contraction. This will not come naturally.
As an aside, this will be the hardest point to get across: there is no formula for adults; only an approach. If it were easy, everyone would be doing it!
I am very happy to expand on these very brief notes, so please ask questions below! Regards to all, KL