I use my 10" chef for anything like that. The knife must be sharp, and it shouldn't be straight downward press, but should still at least start with a slicing action.
If possible, I first try to find the potato's most stable position, which side it wants to rest on naturally. Then I'll get a good start by slicing at least 1/2" into the cut, after which I will place the palm of my left hand on the back of the blade while still holding with the pinch method with the right. At this point I will press straight down, with both hands out of harms way so if anything does slip, all I hit is cutting board.
I use a similar method for halving a squash or melon. When I have gotten a good start on the cut, and it comes time to put full pressure on what I'm cutting, I make sure that all of my body parts are out of the line of fire.