Double-jump

From TF2 Wiki

The Double-jump is an ability unique to the Scout. By pressing the jump key a second time after the initial jump, the scout performs a second jump. The extra height and distance this ability grants can be used to access places which other classes cannot.

Walking off a ledge counts as a jump, which means that you can only jump once after falling off a cliff, for example. Because of that, jumping too late may result in the second jump being seemingly bugged.

Default key: Space on PC , A on Xbox 360, X on Playstation 3

Strategy

In 2fort, use the double-jump to jump from one team's battlements, onto the "roof" above the bridge, and onto the opposing team's battlements.

Double-jumping disregards air physics, so the Scout can quickly shift from one direction to another by holding the respective directional key as you jump. Use this to dodge enemies during combat or to quickly peek around corners with out taking damage.

Use double-jumping to avoid fall damage. The Scout can fall great distances and avoid taking extra damage by using the second jump just before landing.

However, care should be taken when dodging snipers, as a careless double-jump (such as at the peak of the first jump) will leave the Scout falling through the air in a smooth, easily traced path, a prime target for a skilled Sniper.