No confusion here - The HPI diff has two "spyder" (this is one of several names used for the gears) gears rotating around the input gear, hence they are planetary gears. Leadfootdriver is also correct, it's not a ball diff either. A ball diff is what the name implies, ball bearing in the gear riding between slip plates, all under spring tension and adjustable for plus or minus traction.

spyder gears in the HPI Diff - not a single ball in here, if you find a ball in here, you have a problem!

Hope these pictures make sense.



Here is a ball diff



The two above diff systems compensate for inner and outer wheel travel difference in a turn, they are are designed for traction, that is why drifters tighten a ball diff to eliminate this effect.

You can lock the HPI diff with this, or you can fill the diff case with hot glue shutting the gears.