When each battle phase starts, if you have an Event in play, each unit gets "Put 2 damage counters on this unit - > this unit gets Double Strike until end of turn and doesn't untap during your next untap step."
Keywords
Double Strike
Double Strike: An isolated, static ability played after the attack end POP that means, "You may choose a unit that this unit can attack. This unit attacks that unit. Play only if this unit just attacked and only if it tapped for that attack." The unit must tap for the first attack, not the second. If it untaps, it may double strike again. Double Strike doesn't allow you to attack your own units.