balk?

[greymule]

Balk, and even the guys who seldom call balks would call that one. A right-hander does not throw to 1B by lifting his left knee straight up. That's a motion toward home, part of a natural delivery to the plate. Once he does that, he cannot then step toward 1B and attempt a pickoff, no matter how he contorts to do it. He's committed to make a pitch.

Now with a runner on 3B, "straight up" as you describe, might still allow a throw to 3B, as long as the leg movement remains legal (doesn't cross the plane).