Definition: A six o'clock hold, or sight picture, is when you align the sights properly (see how to aim a gun) and then place the target above the sights (or place the sights below the target, if you wish). The point of impact (where the bullet will hit) would be at or near the center of the target.
I dislike this sight picture because the point of impact is an unspecified distance from the point of aim, which makes precise aiming impossible. This sight picture only really works for shooting at one particular distance at the same size target; if you change the range to the target or the size of the target, then the relationship between point of impact and point of aim will change.
I generally prefer the hunting sight picture for all of my aiming.
The six o'clock sight picture is so named because the point of aim is at the very bottom of the target, and targets are usually circular... clock faces are also circular, and 6:00 is at the very bottom of a clock face.
- Russ Chastain