"Relaunch combos:
What you must do is a pop up (launcher) combo and then jump as they arch in the air. Right before they land you have a small window to connect another jump punch that fools the game into thinking they are on the ground, making them stand up and sell the attack standing instead of being juggled. If you mistime it, they will either be juggled by the jump punch, missed, or they will be able to block it. Not everyone can relaunch every character due to collision detection. Kano, Ermac and Human Smoke can relaunch everyone. Sonya, Kabal and Classic Sub-zero can relaunch a lot of characters, and so and so forth. Some characters are very hard to relaunch, some are easy when it's the computer, like the Robots and Sheeva, because they don't block until the combo breaks and their vertical collisions are taller than others. This means they are touching the ground and still hittable for longer duration, but they are in blockable frames longer as well, so ultimately they have just as small a window to legit relaunch. With characters like Sonya, Ermac, Human Smoke, and Kano, you must pause after their pop up before jumping forward since they recover faster than Kabal and Classic Sub-zero. With the latter, you can just hold Up+Forward after the pop up and getting relaunchers with them is very easy. In general, this is another advanced, somewhat cheap tactic, and generally not worth it unless you have mastered the timing."
This was taken from the "General Gameplay Guide" in the Strategy Guides section here at ultimatemk.com.
