its a bodyweight high intensity workout. the design idea is to make your HR get to 90% of max for a short period then allow it to drop to about 65% before taking you back up to 95% again.
the varying hr levels and whole body workload forces your muscles to suck nutrients in to support the work. and the heart really works to keep everything oxygenated.
i will put it this way, i am within the industry and see these things all the time. intensity hasnt received very good reviews from users that i have had responses from. the one that seems to be legit and that i have heard tons and tons of positive response on is the P90X believe it or not. i almost hate to admit that one.