Sneaky Pete says:

The new Bubble Nub Carb Cap is made to work perfectly with any of our quartz bangers (and most other 20mm bangers).  The bubble itself is slightly fatter than the bucket diameter, so it won't drop completely down inside.  Instead, it rests on top of your banger, and you aim the airflow simply by wobbling and tilting it back and forth - no need to rotate.  But our designer went a step further with this bubble cap.  There's no long, clumsy, cheap glass extension sticking out of the top, which is easy to accidentally knock off the banger.  Instead, the top of your Bubble Nub cap has a shaped bead of glass.  Similar to a thumbstick on a videogame controller, it's much easier to guide the airstream just by steering the nub, plus you're less likely to bump it off or break it.

Carb caps are very important to dabbing for a couple of reasons.  Capping your banger limits the amount of oxygen inside the bucket.  This makes the concentrate vapor thicker, giving it more flavor and punch.  They also help the banger retain heat for longer, which is perfect for big rips and getting those yummy low-temp dabs that we all love so much.