If the track they're bringing in is too slow, they speed it up a few percent.
When it starts getting ahead of the track they then slow it down a few percent.
When that setting then causes the track to be too slow they speed it up a little (but not back to the starting speed).
This process is repeated until the tracks are beatmatched, with smaller and smaller changes being made each time.
Each time you make an adjustment and listen to the effect you eliminate another range of the pitch slider and so improve accuracy and time taken to synchronise the tracks.
This is very useful if you have a cheap turntable or happen to use CDJs.