In the context of a Critical Mass
ride: surrounding a car with belligerent cyclist
s) in a manner that obstructs both the flow of riders and the continued mobility of the car.
When this happens, to avoid tacoed wheels and messier accidents from provoked motorists the car-trappers should be gently advised to ride on and the car guided slowly out of the Mass to the next available intersection turnoff.
To get cyclists to move out of your way to let a trapped car out of the mass (safer for everyone involved) a holler of "Homicidal Maniac Driver! Let 'em through!" will generally suffice. To salvage hurt feelings you can wink at the driver for added effect.