The curved toe of the shoe.

[Smitty]

So in other words, instead of standing and watching the free thrower, I should be laying on the floor seeing where the bottom of the shoe actually is touching the floor and where it starts to curve, sort of like a wrestling official looking for a pin? Come on, it's simple..the foot is the thing that can't be over the line. The foot includes the shoe. This is a no brainer.