Refers to the act of using a
swingset. People seem to forget this, but there are relatively few activities more fun than forcefully pumping your legs to use that extra bit of
inertia, to go a little higher, a little faster. It's (
regrettably) the closest you'll ever come to growing wings and
taking flight under your own power.
The feeling of whipping over the ground at high enough speed to turn the wood chips into a grey blur. All the hair on your head and body feeling cool air rush through it. That moment of vertiginous weightlessness as you reach the apogee of your ascent. The satisfaction that comes from seeing the park and the people in it from above instead of from beside. It's not an experience to be missed, no matter how long you've been out of grade school.
Popular variations on the theme are being pushed and jumping from the swing. Getting pushed is cool if only for the bursts of acceleration as your friend runs along with the swing with hands pressed against your back. It's more of a rush than self-propulsion, if not strictly as satisfying. Jumping off is discouraged, since if you're swing fast enough for it to be fun, your probably also going fast enough for it to be dangerous. If there is a suitable wood chip pile to land in, though, those fleeting milliseconds of hurtling through the air (Look ma, no wires!) make it worth the risk.