Limited Slip Action
Think of it like this...
The heavier the silicone oil (diff fluid) you put in the diff, the more it will lock, sort of like changing the spring tension or adding clutch discs in a 1:1 diff for it to lock up more and give more equal traction to both rear tires (or the front as the case may be in an R/C car)...
-Bashing R/Cs since 1996-
HPI Savage S-25, Crawler King, MT2, RS4 Evo, and Sprint 2 Flux
Traxxas Summit, Mustang Boss 302, and Ken Block Fiesta