Here a nice reversal when some tries to mount you from side control.
Knee to elbow
Knee connected to the elbow
Pin the leg
Starting off in your opponents side control.
When your opponent starts his mount, you turn slightly towards him.
Keep your right (inside) elbow deep under your opponent hip.
As your opponent transfers their weight to mount, use your elbow under his leg to lift his leg up...
...now bring your right knee up to touch your right elbow. IMPORTANT: make sure your elbow and knee stay connected! Continue to elevate the leg and turn towards the reversal.
Make sure your elbow and knee stay connected though out the reversal.
Once the reversal is complete use your leg to pin your opponents leg down so you can pass...
...to side control.