Each player decides on their own, EACH round of combat. It works like a dandy. If the question becomes technical, the attacker decides first, but it usually never comes to that.
It protects the players who often get bad dice, or have been diced in the current game earlier. And it allows a surprise element for those whole love to take risks.
The BEST aspect, is unlike LL, which alters the game, the game is not fundamentally changed with Medium luck, because BOTH systems are used so interchangibly, for all due purpose it is still the dice system. If anything, you are only offered a little bit more strategic control on the attack, but you still face dice alot.