It appears in the most recent content update, having restored the ‘quickscope’ feature of the Crossbow, pressing M1 and M2 repeatedly fires the Crossbow at a faster rate than normal, emptying a full clip of both exploding and piercing bolts in under 2 seconds.
To those with different key bindings… firing the Crossbow once, then immediately scoping back in allows one to shoot another bolt without delay. Only works in multiplayer.