Opening the closed guard in No Gi grappling is trickier than with the Gi. You don’t have much to hold on to, and the opponent is slippery.
Marcelo Garcia showed a brilliant way of opening the guard that he used agh the top level in ADCC competitions.
With this set up, your opponent can not grip your leg and sweep you.
Check it out:
Sloth Jiu-Jitsu: you can be slow and unathletic and still kick butt in Jiu-Jitsu.