Having a smaller blast radius than a Thermal Detonator, the Impact Grenade explodes directly on contact with any object, as the name suggests.
Since the Impact Grenade has the smallest blast radius of any other grenade, it can be very difficult if the player is using the grenade at enemies from far distances, because the damage could be much smaller than expected. If the enemy is only within 1 meter, they'll only get damaged by a small amount.
Despite the small radius and will explode upon impact, the grenade can do more punishment than the detonator by 30 more damage. It also can be used again after 20 seconds.