With 5 points in Vengeance’s “Rapid Strikes” node, when you cancel Warpath with Vengeance it will use Vengeance twice with just a single press of its hotkey before resuming the spins. Removing that 5th point in Rapid Strikes brings it back down to 1 attack before Warpath resumes. This is done with Warpath held down the entire time.
edit: it seems to just be anything beyond a certain amount of attack speed for Vengeance, maybe 20%. Doubles the input if you keep Warpath held down.