You mean turning vsync on helps with screen tearing. Screen tearing is more likely to happen above the monitors refresh rate. You can still have tearing when you are below because the gpu just outputs frames whenever it finishes unless it is synced.
Active sync? Wrong subject, I think you mean freesync. Instead of the gpu syncing to the monitor refresh rate which is what vsync does, gsync and freesync sync the monitor to the gpu. This way you don't have the gpu waiting on the monitor so you don't get input lag.