The Green Shell is mainly used for as a basic offensive Item. A racer can throw them in front of them at an opponent, and it will speed through the track in a straight line until:
A-It collides with other driver, in which it will knock them over. They disintegrate instantly upon impact.
B-It collides with an item trailed by racers. Items spinning around racers or on track, like a banana, also destroys it.
C-It comes into contact with the explosion impact from a Bob-omb or a Spiny Shell, or the sound waves from an active Super Horn.
E-It is hit by throwable items from racers, like Boomerang, Fireballs or shells (all 3 variants).
F-It ricochets off the walls five times, the fifth one destroying it (In Mario Kart 64, it can bounce an unlimited number of times.). This makes them especially dangerous in tight spaces.
or G-It falls off the course.
Due to how they work, it is a great item to be used for taking out the racer in front who is currently trailing their items, unlike the Red Shell.
Green Shells can also be held behind a Kart to act as a defensive Item against other enemy Green Shells or the more powerful, homing Red Shell. They can also be shot backwards, acting the same way as they do when they are launched forward. This technique is also great to take out racers who are attempting Drafting.
The Green Shell is also included in a Lucky 7Item in Mario Kart 7 as one of the seven usable Items circling around the racer. It has the same effect as a regular Green Shell, and can be used when it is in front of the Kart, or simply to ram into an opponent racer to knock them over.
In Mario Kart 8, the Green Shell is now an Item member of a Crazy 8Item. Once again, it's one of the Items, now eight of them, as the name implies, spinning around the driver and can be deployed when it is in front of the racer, or crashing into another driver.
In Mario Kart 8 Deluxe, forward thrown Green Shells are believed to consistently ricochet off walls in a weird way to hit racers driving by them. This behavior is called Smart Bouncing by the community, and this behavior makes Green Shells a pain to dodge should the track does not provide enough space for the racer to get out of the way. However, this has not been proven and remains speculation.