Prolonged Rocket Boost (PRB) is a technique that maintains max speed over any type of terrain. There are only two ways to start PRBing: 1) a perfect start boost, or 2) a rainbow zipper boost. The only way to maintain PRB is to never have a gap between your mini-turbos (MTs) or the original boost. This means that you must start and complete your next MT before your previous MT wears off.

The length of time a MT is active is only dependent on your kart's acceleration stat. So, the Dry Bomber is the easiest kart to maintain PRB because it has the highest acceleration stat. Egg1 is a little bit more difficult, Poltergust still more difficult, and the real all-stars of MKDS can maintain PRB in the ROB-BLS, which is incredibly demanding.

When you lose PRB in a run, the only way to regain it is by hitting a zipper and then continuing to MT. PRB cannot be started by using a mushroom boost, and you will lose PRB when you use a mushroom. PRB is useful for many tracks in the game, and the best tracks to practice the technique are probably Retro Mario Circuit 1 (to maintain start boost) and Retro Luigi Circuit MKDD (to maintain zipper boost). If you're really talented, you can even sneak some correction hops in between MTs. Remember, you're going max speed the entire time you are PRBing, so your racing line is what separates good times from great times.

Little tidbit: although Dry Bones is typically the character of choice in time trials because it's the least affected by terrain slowdowns, with PRB every character goes the same speed given the same kart.