Moving the pitch adjust will slow the track down or speed the track up. This allows you to take two tracks that are of different tempos and mix them together in time. The pitched BPM and the pitch % are shown on the Virtual Deck.
The simplest way to adjust the speed to match the other track is to move the pitch the BPM value is the same as the track that is already playing. By repeatedly playing from the temp Cue Point and ne tuning the pitch slider you can get the tracks playing at exactly the same speed.
NOTE: Serato DJ Pro will play music at a range of speeds with or without key lock.