A Landing Boost can be performed (with the "Landing Boost" tuning part equipped) by ensuring the vehicle lands from an at least 2 Airtime long jump with all its wheels touching the ground simultaneously. Successfully performing this move will be signified by a sudden "dash" forward of speed by your vehicle, as well as a discharge of sparks from your wheels and a distinctive sound. If you combine it with "Jumpshocks" and activate them at the right moment, it also gives you a boost up and you can speed up your momentum for a longer time by perfectly performed jumps.
A visual example is included below:
Note: The tuning part Landing Boost can only give a forward boost to your car when you hit a perfect landing!