Share 'What constitutes a Foot Foul???????'

I was watching the Music City Open on youtube last night. Barry Schultz called Nikko Locastro out on a foot foul. It was an inside the circle putt. I NEVER saw EITHER of Nikko's feet leave the ground after watching over and over. But, someone in the group called the second on Barry's call, and Nikko had to re-putt.  I looked in the PDGA Rulebook, but I didn't see exactly what constitutes a Foot F…