Left hand rolling fingers

[DepressedNazgul]

When you play 2 notes on adjacent strings on the same fret, you roll down your finger to make the 2nd note sound. It seems pretty easy and I use it alot, but I can't do the opposite thing - For example, playing string 2 fret 3 and then string 3 fret 3... How do you roll your finger upwards?

[Sentinel]

I bar my finger across both strings just before rolling it. But I don't put enough presure on the 3rd string for it to sound out. As I roll off the 2nd string, I only raise it slightly so it mutes that string while I play the 3rd string.