To unlock the Feeling Dizzy? trophy in Trials Rising, you will need to perform at least seven flips on one jump.
A good track to do it on is Yellowstone Caldera. Shortly after checkpoint 7, you will travel down a large ramp. Instead of racing down the ramp and performing a large jump, ride down slowly and drop off the end onto a smaller ramp with a water jet at the end of it.
Ride into the water jet and hold backwards to start performing backflips.
You can watch a video below (Credit to Maka91Productions on YouTube):