It's been said before:
JUST enough to keep the left arm straight.
....but not so much that you overly inhibit motion.
...
Now, I have a question I've not really thought about until now....mainly cause I'm fooling around with Hitting now....and with lots of success actually....you guys may have a new "mouth breather" on the board
....
Ya....would a Hitter have more Extensor Action? (a stronger pull)...
Seems so to me....to promote structure and discourage stroke length (stop at Top ["3/4 position"] not End).