I think what TripleOne is referring to is not the split bottom mount… but rather the practice of catching the string with the non-throw hand.
My take is that this is a quick-easy. way to stabilize the string and yoyo. (similar to intersecting the trapeze close to the yoyo to maximize control) Catching the string half-way down with the non-throw hand provides control you would not have otherwise.
In the case of a front-throw it allows for an easy transition into either the split-bottom mount or a bind.
In the images below you see these guys doing (basically) the same thing as they prepare to do a horizontal grind.
I started catching my string this way and (I believe) my transition from my throw into tricks and binds was more quick and crisp.
https://www.youtube.com/watch?v=0NsLXaOhJ2o @ 0:24
https://www.youtube.com/watch?v=tzJ8WNqZKJI @ 1:55