"Yoga is a discipline that succeeds brilliantly at smoothing the ups and downs of emotional life." - William J. Broad, The Science of Yoga
The secret I've learned after a decade of practicing and teaching is that flexibility is relaxation. And you relax more deeply after being stressed. Learning how to articulate and regulate your muscle can only be completed with knowing how to deregulate it. You can do Sun Salutations everyday for a year and it will not make you any more flexible. It'll just make you become really good at doing Sun Salutations. Those who practice only active yoga most likely do not know how to deregulate fully while awake and alert. Even in an active standing pose you need to know which muscle to tighten, which to relax a bit, and which to relax completely. If you don't know how to deactivate it's impossible to know how to activate properly.
The breath moves along the spine. It massages all of the internal organs from the pelvic floor to the top of skull. If you aren't breathing steadily, deeply, and into the right places your vital organs are not in an optimum condition and you are not in an asana. The breath is constantly creating the ups and downs of the inner life, literally. Yoga is the smoothing of its ups and downs. Relaxing the right places -- sinus cavities, inner mouth, throat, and chest cavity allows you to tighten voluntary muscle without obstructing the involuntary muscle and the movement or functioning of the internal organs. The inflexibility isn't in the muscle. It's in the connective tissue, the nervous system and the vital organs. Soften the muscles and you give them the space to breath and be flexible.
Static passive stretching in passive yoga of yin and restorative is the opposite of static active stretching or dynamic active stretching in active yoga. It's a shift from anti-gravity to pro-gravity which took me at least a year to fully differentiate. When newcomers show up in my yin class they often remain in a flexed position as I instruct them to soften around the joints and relax deeply. To increase a wider range of motion you need to deactivate your muscle completely while compressing and extending your joints gently. Flexibility is the ability to flex deeply and the capacity to release the flexion completely. Yoga is first and foremost a discipline of emotional flexibility which includes physical and mental fitness. Knowing how to surrender and deregulate opens you up to a wider range of motion everywhere--body, heart, and mind.