Most people love doing jump kicks because they are so dramatic. But you give up control of your balance by being in the air, so it isn't something you want to do all the time. Also, jump techniques require a large motion, making them potentially easier for your opponent to read. By the time you try to make contact, they may have already gone home!
There are 2 main important points to consider when doing jump kicks:
1. Setting up your body posture before jumping
2. Timing the jump correctly.
Usually, the best time to jump is after executing a setup technique(s). Doing this puts the opponent behind your timing and allows for an effective jump attack.
Jump Kicks #1
Tobi Hiza Geri (Jump Knee Kick)
This is one of the many ways to execute Tobi Hiza Geri in Jissen Kumite.
Remember that technique by itself is not enough. You need to practice all basic techniques with stepwork and timing in order for them to be effective!
REMEMBER: You need to practice over and over again--not just in your head. Receiving partner should also move with different timing to challenge the attacker.
Train safely! OSU!