This is a guess:
I think his riffle shuffle starts out as a true shuffle, but as he bridges them for the cascade, diagonal pressure is applied causing one half to fall about 1/3 to 1/2 inch towards his body at a slight angle.
The right thumb can hook the upper half(which is on the bottom), and pull it through SIDEWAYS in a manner similar to a traditional tabled push-thru shuffle, and cut it to the top.
His hands are so big that it's easy to hide this. That is my guess, and that is based on observations of a friend of mine that DOES do a shuffle that way and his looks very convincing, better looking than the Heinstein shuffle, in my opinion.