The card states: this extra damage resolved before normal combat damage is assigned. What if this extra damage KOs the blocker? Can the blocker assign it's combat damage if the extra damage is enough to kill it before combat damage is assigned?
Senario 1: Quicksilver attacks and is blocked by a sidekick. The effect triggers and the sidekick dies. Since the sidekick dies before he can assign his damage, Quicksilver lives. Quicksilver is returned to the field zone and lives to fight another day. The sidekick is moved to the prep area. Quicksilver is still blocked, so no damage goes to the opponent.
Scenario 2: Quicksilver attacks and is blocked by a sidekick. Sidekick dies from the effect, but still gets to assign his 1 damage. Quicksilver also dies. They both move to the prep area. Quicksilver is blocked, so no damage goes to the opponent.