This is the basic mechanics of an over the knee spanking.
Based upon what tool you intend to hit with and how much physical damage you wish to inflict, it may change the way you sit, your grip, and where you should place the target (e.g. the sub's ass).
For a heavier-weight tool such as a wooden paddle or large hair brush, you will probably use a standard grip in most situations. For deeper bruising you are best off using a legs forward position.
For a lighter-weight tool such as a ruler, slapper, wooden spoon, slipper, or strap, you can use an overhand grip if you want to deliver quick, repeated blows with a satisfying smack on impact. You can use a standard grip if you want to hit deep and cause bruising. Use legs back for the quick stinging blow and legs forward for a deeper penetrating blow.
*A quick note: focusing on pulling the bottom of the handle will make the striking surface of the tool move faster than if you try to move the striking surface.
As you can see the mechanics are similar for both strikes with a couple of small differences. With a stinging blow the focus should be on quickness instead of power (the power is generated by physics rather than muscle strength) and you stop upon impact rather than continuing to "pull through" the hit.
One other difference of note is that you may have better luck shifting the target position depending upon what style of hit you are attempting.
For penetrating blows the target should be placed (on your lap) along the right side of your body (assuming you are right handed, switch to the left side for lefties).
For stinging blows performed with the overhand grip (and sometimes penetrating blows with the overhand grip), you may find you can hit harder if the target is placed (on your lap) at the center of your body.
One way to practice these techniques is to take something like a wooden spoon and try to perform the motions while standing. You are better off staying as relaxed as possible until you near the hit point. You can adjust things as necessary. Your goal with the spoon is to make the "swish" of the air be as high pitched as possible, since that means the head of the spoon is moving faster. If it makes something like a whistling sound, you are generating tremendous speed.