Why lower it at all?
Water left in the skimmer throat and fittings can freeze, expand and crack them. Lowering the water a few inches below the skimmer mouth leaves nothing there to freeze. A winter skimmer plug or cover plate seals the opening instead, which is why a pool with one can stay full.
How far is a few inches?
Guides range from about 3 inches below the skimmer to 6 inches below the lowest opening; 4 to 6 inches covers most of them. Do not go further than you need to. Most winter covers allow for about 12 inches of lowered water, and past that a cover can come up short.
Why you never drain an above ground pool
The walls of a hard-sided above ground pool are built to lean on the water inside. Drain it and nothing holds them in shape, and an empty pool can collapse, especially in wind. A liner left exposed on a warm day can also shrink past use. Keep the pool at least half full, and in practice all but a few inches.
Intex and Bestway pools
Soft-sided and frame pools are the exception. Intex says to drain and take its pools down once temperatures drop below 41°F, and Bestway recommends taking its frame pools down for winter.
The full winterizing guide covers every step.