When you animate a walking character and lock the feet down, the locked position will be perfect on the key poses. However, when you use motion keyframes to auto in-between the animation, you'll notice a movement of the feet. This motion is caused by the in-betweening of the rotations.
The nail holds in place the body part on the keyframe, but it will not hold it for the entire duration between keyframes.
If you place a nail on the shoulder to move just the arm, you don't necessarily want to keep the shoulder in the same place until the next key pose. You will probably want the shoulder, or any other part that is nailed, to have a slight, natural motion following the main body action. Of course, this is not the case when you fix a hand or a foot on the spot. If you have a fixed hand or foot, you will probably want it to stay exactly where you placed it. That is why you can apply an IK constraint on a frame range and on selected body parts.
To fix the part on the same spot requires an angle (rotation) correction on the parts that are moving too much. For example, to fix a foot that is moving too much, you must correct the angles on the foot, leg and thigh. To fix a hand, you must correct the angles on the hand, forearm and upper arm.