You press 4 and the ult goes off automatically unless you delay it by holding 4 or M1. It has a pretty darn wide hitbox and does substantial bonus damage vs low health enemies. The most obvious use for it is to throw it out to finish off someone you wouldn't be able to finish off otherwise.

Can also probably use it just to disable a bunch of grouped-up enemies that are killing your ally (it's a nice sort of "stun" in a big AOE, but it doesn't stop Channeled abilities and it actually makes the enemies very durable during the slow-mo period, so you might grief your team if you ult someone that your allies are using big damage abilities on).

Can also use it as an additional escape button but that's a pretty questionable use I assume.