The best kind of swing push, if used sparingly, this describes when the pusher takes a kind of running start at the pushee such that the momentum carries the pusher so far and the swing so high that the pusher actually runs beneath (or ducks under) the person they're pushing.

The recipient will demand more, but this coveted push is best used no more than once per swingset-session, so as not to devalue its thrill factor and drive little kids to extreme sports at unripe ages.

Apparently, the term is also used to describe ducks low in the notoriously fierce duck pecking order. Who woulda guessed?