Stunt Jumps are hidden ramps propelling Vehicles into slow-motion jumps in GTA 5. 50 Stunt Jumps exist in San Andreas. The Stunt Jumps Collectables are required to reach 100% game completion and will also earn you the "Show Off" trophy & achievement.
When attempting a Stunt Jump, you are required to land in a specific spot and with all wheels on the ground. During the jump, adjust the vehicle in mid-air with the Left Analog Stick so that it lands properly in the designated spot.
It's best to attempt these USJs as Franklin especially with his Ability to slow down time. A recommended vehicle for these jumps would be the Pegassi Bati 801 motorbike, although some stunts are easier and safer if done with a sports car.
If the vehicle is too badly damaged, simply switch to another character and then quickly switch back to see that the vehicle has been magically repaired!