Power Yoga is a general term used in the West to describe a
vigorous, fitness-based approach to vinyasa-style yoga. Most power yoga is closely modeled on the Ashtanga style of
practice. The term “power yoga” came into common usage in the mid
1990s, when several yoga teachers were looking for a way to make 
Ashtanga yoga more accessible to western students. Unlike Ashtanga, 
power yoga does not follow a set series of poses. Therefore, any
power yoga class can vary widely from the next. Two American yoga teachers are most often credited with the near 
simultaneous invention of power yoga: Beryl Bender Birch, based in New York, and Bryan Kest, based in Los Angeles. Not coincidentally, both these teachers had studied with Ashtanga master Sri K. Pattabhi Jois. Using the term power yoga differentiated the intense, flowing style
 of yoga they were teaching from the gentle stretching and
meditation that many Americans associated with yoga. Another name
 often associated with power yoga is Baron Baptiste. 
Baptiste has his own method, which is only taught by teachers he
certifies.Yoga description reposted from this  website: www.santosha.com