With a couple of exceptions, there's no collective or blade angle change in an autogyro, and whereas the autogyro runs in autorotation at all times (and flares by changing rotor disc angle, not by pulling collective (altering all blade angles simultaneously), and whereas the autogryo rotor disc has no connection to a transmission or engine, you can't correlate the autogyro with the helicopter.

The helicopter collective must be lowered and the rotor clutch disengaged (manually or automatically, depending on the system), to prevent loss of RPM (which will occur if there's drag on the rotor disc through mechanical means). flare at the bottom end in training will be power and collective, and in an engine out, final flare involves use of the collective.

Rotor blade inertia is crucial in a helicopter as is RPM. Increasing blade angle by pulling collective when in an autorotative state will decrease rotor RPM. This is not an issue with the autogyro, in which use of the cyclic to vary the plane of the rotor disc is the only option, other than adding power through an independent, fixed pitch propeller. (in nearly all cases)