Blow-back is the rearward tilt of the rotor disc during the transition to forward flight.
Blow-back is due to the combination of flapping and the transverse flow affect. In forward flight, the advancing blade produces more lift than the retreating blade. Flapping is used to correct for the dissymmetry of lift across the rotor disc. As the advancing blade flaps up, it causes the front of the helicopter to rise. In addition, the induced flow will be greater at the rear of the rotor disc during forward flight, increasing the lift on the front of the disc, thus increasing blow-back. As forward movement occurs, the pilot will have to continually increase forward cyclic to compensate for flap-back/blow-back.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-19
Principles of Helicopter Flight, 2nd Edition, pg. 94
FM 3-04.203-2007 Fundamentals of Flight pg. 1-40