Dealing or receiving damage from any source (including champions, monsters, towers, and minions) counts as combat and will result in the base movement speed bonus of 25.
There is no range limitation on this damage.
The application of any non-damaging combat debuff (such as exhaust) to an opponent will trigger this state as well.
If invulnerable (such as being under the effects of Kayle'sIntervention), then autoattacks and non-cc abilities will not trigger this state.
Heals and buffs to allies are not counted as combat.
Damage-over-time spells and abilities reset the timer with each tick of damage.
This item is especially useful if you don't have a Teleport to switch lanes.
This item is good in particular early-mid game on roaming champions, that want to quickly gank different lanes. They lose most value when team fights start to happen. It also allows for chasing and escaping.
When escaping, remember that poking (e.g. Ashe'sFrost Shot) will reset the movement speed, however, healing yourself (e.g. Taric'sImbue) will let you keep the 105 movement speed buff.