The Grenade Jump is a technique in the Team Fortress series. To perform a basic grenade jump, the player must jump near a Frag Grenade or MIRV Grenade when it explodes. The explosion will shoot the player a great distance, depending on where the person is in relationship to the grenade when it explodes.
There are two types of Grenade Jump; Hand-Held (or HH) or dropped. A HH grenade is done by holding the grenade, then jumping just as it explodes. The dropped grenade jump is done by dropping the grenade, then jumping forward near the grenade as soon as it explodes. The HH jump is easier to do, but it incurs more damage, while the dropped jump is harder to do, but can fling a person further and faster and does less damage if the player doesn't jump right next to the grenade.
The Grenade Jump is very useful for classes that don't have a way to quickly move across a map, such as the Spy and Pyro. However, remember that you will suffer some damage when doing it, so don't expect to be able to take a lot of damage after doing a grenade jump, especially if the class has low health and/or armor.
When combined with Bunnyhopping, the grenade jump can get classes to base entrances that they usually can't access, such as the top of the towers in 32smooth. This gives classes like the Spy a major advantage, as most people don't expect such classes to come into a base from such entrances.
The Heavy gets much less trajectory from a grenade jump because he is moved less from an explosion that the other classes are. Therefore, grenade jumps shouldn't be performed as a Heavy because it'll give you less trajectory that a grenade jump from another class and will inflict a decent amount of damage.