Hi Joe,
He explains it pretty well in the video. He cut the clump of hair from the heel of the foot. He didn't want a lot of the finer fuzzy underfur that's right next to the bone, and he didn't want any longer stray guard hairs sticking out either. He just wanted to use a nice compact clump that was about the same in total length. If he kept too much of the underfur, it would be a little too bulky for that sized fly. If you look close, he is useing some of the underfur and the shorter more even guard hairs in his clump.
Regards,
Mark