Attacking a moving builds Energy stacks. At 100 stacks, your next attack is Energized.
Energized attacks heal you for 3 - 60 (+0.30 bonus AD, +0.40 AP) and grant 30% increased Movement Speed for 1s.
Healing is 60% as effective when used on a minion (30% for ranged champions).
If the attack that activates Fleet Footwork is a critical hit, Fleet Footwork's healing is increased by 40% of your critical damage modifier