A recovery boost is a technique featured in most Mario Kart games since Mario Kart 64. After falling off the track or going out of bounds, Lakitu will fish a driver out and drop them back on the course. If the player taps the acceleration button as soon as their driver hits the ground after being dropped, they will receive a small boost to get them going- about the same as a mini-turbo. This is very helpful to avoid losing further time after a recovery since unfortunate players would need to accelerate from zero, which can take a while for heavier karts. Sadly, this technique was removed in Mario Kart 8.
R.I.P. Recovery Boost