In reference to siren's writeup, I just have one thing to add as a Canadian west coast climber. Over here, what you deemed an "open crimp" we call an "open hand" grip. Crimp implies just the "closed crimp" you talked about.

Open hand grips are typically weaker than the closed crimp you talk about, but that is not always the case. A friend of mine, when we boulder, can always hit long slaps in a full crimp (crimp is taken to be a closed crimp in this case to avoid confusion). He doesn't need to reset his grip or anything, he's already in a full crimp. However, when I attempt the same move, I can never hit it in a crimp. I always fall out to an open hand grip, then when my body stops swinging around I switch to a crimp grip (closed).

Such is life I guess. I blame the campus board for my open hand grip being the unconscious preference.

Note: NEVER campus in an closed crimp grip, unless you enjoy grinding your knuckles out of existence in the space of an hour.