What is a landing burn?

By this point the booster is nearly empty and therefore light. Even throttled all the way down to about 40%, a single engine still pushes harder than the empty booster weighs — so lighting it does not slow the rocket to a hover. It slows, stops, and starts climbing again.

The burn therefore has to be timed so that speed and height reach zero at the same instant. Light too early and the booster stops short and falls back; too late and it does not stop in time. There is no hovering to correct a mistake and no second attempt, which is where the nicknames hoverslam and suicide burn come from.

On a Falcon 9 it starts around T+7:52, at roughly 8 km up and about 1,100 km/h, and touchdown is at about T+8:15. Twenty-three seconds to go from faster than an airliner to standing still.

The legs come out during the burn rather than before it, because deployed legs in fast air would be torn off. The grid fins have been steering the whole way down, but they stop mattering in the last few seconds — at the end the engine is doing everything.