Rage spell is an offensive card best used for swarm decks and slow attacking/moving troops like Balloon, PEKKA, Giant and Royal Giant. It increases movement speed and attack speed drastically by 40%. It has long duration and a very large area of effect, which can be easily placed well.
Spawning cards like Goblin Hut, Barbarian Hut, Tombstone and Furnace makes the production of troops faster while in the effect of Rage spell. Buildings like Tesla, Cannon, Inferno Tower, Mortar and even the Towers are also affected by the attack speed increase.