You can lock (synchronize) up to 4 videos at a time.
To lock 2 videos, put the Analysis section in 2 screen mode. Open 2 videos and get them to a common position (such as impact in this example). Move your mouse to the top to drop down the Menu Bar. Click on the Lock icon and you will actually see it lock . Your 2 videos are now locked together and will scroll through the frames in sync.
Click the lock again to unlock the videos.
You can individually lock 2 more videos by putting the Analysis section into 4 screen mode (see below). Open 2 more videos and get them to a common position. With one of the bottom windows highlighted, move your mouse to the top to drop down the Menu Bar (the lock will now be unlocked again). Click on the Lock icon and the bottom videos will now be locked in sync. If you look at the lock again, you will notice that it looks like this to show that there are 2 sets of locked videos.
To lock ALL 4 videos together, simply click the lock again. The lock will now look like this and all 4 videos are locked in sync.