Video sync puts audio and video in perfect sync for you to see what's going on.
What happens technically is that latency is added to your audio, so that the much slower video can catch up and the two can be synced together.
This, of course, means you can no longer play together as the latency is much too high, so be sure to turn it off when you start playing together!
Get to it
1) To be able to enter the Video Sync mode you first need to enable the feature. To do so, please go to Settings - Advanced Settings - Video Advanced - Video Sync.
2) Now you should be able to see the Video Sync button in the bottom row.
3) To enter the Video Sync mode please click the Video Sync button when in a session. Please be aware that when going into the Video Sync mode EVERYONE in the session goes in Video Sync mode.
4) To leave Video Sync mode just hit that button again.