Where is the center swivel of a Ching sling supposed to mount? Some specific part of the distance betweeen the front and rear swivels, near the center of gravity, ?

I have an old falling block 22and as a kid I couldn't figure out what the third swivel was for . Many years later I found out .Mine has the happens to be at the balance point but certainly that's the most convenient point also.It's on the left side.