Beatmatch
Matching the speed and beat positions of two tracks with your own hands. You bring the BPM close with the tempo slider and close the remaining drift with NUDGE. The result is the same as SYNC, which the machine does automatically, but the difference is that you tell by ear and correct it.
How to beatmatch
① Listen to the next track in your headphones and set a CUE on a downbeat. ② Start the next track in time with the downbeat of the track that is playing. ③ Use the tempo slider to bring the next track's BPM close to the playing track's. ④ When the beats start to drift, push the track slightly forward or back with NUDGE or the jog wheel to close the gap.
Repeat ③ and ④, and once the beats stay together after listening for a while, you have beatmatched.
Tips for hearing the drift
You can tell whether the tracks are off by how the kicks overlap. When they are aligned, the kicks sound like one sound; when they are off, they split into two: "ba-dum."
The hard part is telling which one is ahead. If the next track is ahead, hold it back; if it is behind, push it forward. If you are unsure, move it a little one way, and if the gap widens, go back the other way. Keeping the headphones on one ear and listening to the room with the other makes it easier to compare the two sounds.
Why practice when there is SYNC
Press SYNC and the machine matches the speed and beat position for you. Using SYNC on a gig is fine. The reason to practice beatmatching anyway is so that you can fix it by ear when the machine gets it wrong.
If a track's beatgrid is off, SYNC will line up to the wrong position. If you can hear which track is off and by how much, you can fix it on the spot.