Rocket uppercut is a movement technique that propels Jak vertically while maintaining a grounded state. The technique has applications in many speedrun categories and is one of the key components in Lava Tube Skip. A rocket uppercut is performed by uppercutting into a small piece of vertically oriented ground collision. During the uppercut, Jak makes brief contact with the vertically oriented ground collision which sets him to a grounded state. This brief contact also amplifies the vertical momentum from the uppercut, similar to the amplified horizontal momentum of a boosted uppercut. The end result is Jak flying up into the air in a grounded state able to perform a limited amount of movement options before descending and entering his falling state.
Rocket uppercuts can also be performed by certain cutscenes interrupting the uppercut instead of a grounded state, such as Yellow Vent Uppercut.
The rocket uppercut sometimes occurs during casual playthroughs so it is impossible to determine who the first one to perform a rocket uppercut was. The first person to utilize a rocket uppercut in speedruns was Billbraid when performing an older variant of Red sage cutscene skip.
Some textures within the game are programmed to have ground collision for walking on, such as grass and wooden planks. While these textures were intended for horizontally oriented use only, they were reused when designing vertically oriented landscape details, such as grass hanging off of a cliff. The wooden planks in Volcanic Crater are programmed with ground collision so Jak is able to walk on them, but this is the case for all sides of each plank, meaning Jak can get a grounded state from the top, sides, and bottom of each plank. This oversight allows for rocket uppercuts to be performed off of the sides of the wooden planks.