The portion at the beginning of a Critical Mass
ride when there aren't yet enough cyclists to comfortably / safely "take" a street. At this point you could say that the group is especially critical and not really very massive at all.
Activities taken at this stage often discussing possible routes, gathering some demographic data (how did you find out about this ride? Have you been on a CM before?), exchanging tips on safe riding and how not to get ticketed, promoting other cyclist / activist events, sharing stories of cycling glory (cross-country trips) and infamy (particularly gory accidents witnessed), commenting on each other's bike gear, making suggestions for how to solve particular equipment problems, and an awful lot of cycling around in small circles looking at one's watch.
In Critical Mass Vancouver, it also involves waiting for the Fossil Fools to put on their eerily accurate velociraptor costumes and occasionally hammering out contingency piggyback plans with members of Food not Bombs and/or Adbusters.