Re: reverse breathing
I think the physical part more or less defines reverse breathing: contract abdomen when breathing in, expand abdomen when breathing out.
But depending on the goal of the practise, I think the visualisation, intent and other details differ from art to art, teacher to teacher or even from one exercise to the next (around 0:25 Bill Gleason says he will show a more complicated version the next day).
I don't know Bill Gleason personallly, but I hear and see him explaining that in this variation you visualize drawing energy up from the earth through the legs into the hara when breathing in and sending energy out through the palms when breathing out. He explains this variation is for building ki.
I was taught a different variation for body conditioning (it could be related to the variation Bill Gleason is teaching in the video, but I'm not sure).