Higher = fewer fumbles.
At 79 I saw about half the fumble rate (by % of snaps) of the NFL. Lowering to 75 to get a few more fumbles and will probably go lower. I'm a little sensitive to TOs until INTs start to decline.
It's counter-intuitive, I realize. Think of the slider as the "hold onto the football" slider. You can find the tooltip explanation in the Main Menu, and my data backs it up.