The Insert Bar 1 button (similar to the Insert Pickup Bar button):
It’s often difficult to sync the very first beat of a song, because there’s nothing in the recording that tells you when it will start – there’s just some short length of silence before the music begins. When this happens, you can simply ignore Bar 1, and start syncing from Bar 2 (though the list will display this as Bar 1, initially). When you’re satisfied that your Bar 2 (the list’s Bar 1) starts correctly, click this button to create a new Bar 1, which will start one bar earlier than the first one you synced. If it’s not perfect, you can tweak it with manual editing, but if you’re careful, it’ll be easy to get it right..