This allows you to beat match tracks on the Virtual Decks, and then instant double the track down to a slot, ...
***MOST IMPORTANTLY, freeing up the Virtual Decks to continue your mix with other tracks.
To do this is Serato, you click and drag an already playing track from a Virtual Deck or sample slot and drop it on another sample slot to begin instant doubling. You will need to ensure you have Instant Doubles enabled in the Setup Screen.
This would be a really nice feature to add to Rekordbox if it can't already do so..