Rumble shoots a taser that deals 60 / 85 / 110 / 135 / 160 (+40% of ability power) magic damage and slows the target hit by 15 / 20 / 25 / 30 / 35% for two seconds. Rumble gains a charge of Electro-Harpoon every 6 seconds and can store up to two charges at once. Each use of the skill generates 10 Heat. If the second taser slows an enemy that has been slowed by the first, the slow is refreshed and its movement speed reduction is doubled.
While in the "Danger Zone", the damage and slow percentage are increased by 50%.
Rumble can still use Electro-Harpoon if he is overheated as long as he has a charge.
This is the reason I main Rumble, sort of how peopke like Fiddlesticks and Rammus, they have that one ability that just annoy's the living hell out of people.