Strength builds on strength. Use strength to consciously squeeze energy in and get even stronger. But Dandayamana-Bibhaktapada-Paschimotthanasana (Standing Separate Leg Stretching pose) teaches that using strength also increases flexibility.
Reciprocal Inhibition makes it happen: engage muscles on one side of the body, and muscles on the other side more fully and safely release. Feel it in legs and arms, but also core and back, shoulders and chest. Intentionally pack energy in on one side and feel it release on the other.
It works in the body, and strength builds on strength in mind, emotions, and spirit, too. Play with that.