Well . . .
I think this probably sums it up . . . 

Control Through Communication

Have you ever thought you could 'control' a horse by holding onto
it's leg ?

I'm sure you haven't . . . nor have I !

So the idea of holding onto your horses leg whilst it has no halter and lead rope on seems even more absurd, especially if it was standing on a box just to add to the degree of difficulty !

It sounds neither safe nor achievable . .

And of course it is neither safe, achievable or ethical to control a horse by trying to use strength or intimidation to gain control

However, as this picture shows . . .

Understand what motivates your horse, Understand innate horse behaviour and its application to training . . . 

And the impossible can . . . And does happen . . .

whatever you choose it to be.