One extract I found. Not as technically accurate as I would like but it's a start while I look for other references:
'Phase lag will cause the extra lift to be seen approximately 90 degrees later in rotor rotation in semi-rigid two bladed rotor systems. Phase lag is a separate phenomenon from gyroscopic precession, which applies only to rigid systems. Rotor systems are not rigid systems since all helicopter rotors are designed to "flap" up or down as they change position around the rotor arc. This flapping counter-acts dissymetry of lift in forward flight. Phase lag is a property of all rotating systems acted upon by a periodic force. For systems hinged at the axis of rotation ( in our case, a semi rigid flapping type rotor head) the phase lag is 90 degrees. For systems that are hinged at some distance from the axis of rotation (such as a fully articulated rotor head) the phase lag is less than 90 degrees.'
Sorry ELT2GO, we seem to have drifted.