When I first started Power Yoga, it was from the book written by Beryl Bender Birch, titled “Power Yoga.” That was a description of the primary and secondary series of Ashtanga yoga.
As yoga evolves (and this is normal – about half of the asanas we do today weren’t around 40 years ago), Ashtanga is still power yoga, but power yoga has broadened to include more postures and different sequencing.
It can be a very good way to drop into the stillness of the mind when working the body very hard. Each pose = 1 or 2 breaths, usually moving large muscles for endurance and strength.
one should understand, what different names we give to Yoga, the ultimate objective of practice is SAME -Equanimity of mind. Asanasa are just the primary step so that it helps you to get rid of body problems and thus climb up the ladder of mind relaxation. But unfortunately, across the world people get stuck at the first step of Yoga as different postures, speed and strength and all are unknowingly practicing the physical aspect of Yoga Only…
as this is a hybrid created by the market to make yoga more mainstream, it really does not have a set definition except to focus more on the strength component required for yoga and thus modeled on the ashtanga yoga style.
Ultimately it is whatever a given instructor has decided it to be.