Transverse flow is the decreased lift at the rear of the rotor disc due to an increase in induced flow as the disc moves through the air, producing a roll to the right.
Transverse flow occurs as a result of forward flight or a significant wind. As the helicopter moves forward, the airflow at the front of the disc has not started its downward flow. At the back of the disc, the induced flow or downwash is more significant, which reduces the angle of attack on the blades. As a result, the front of the rotor disc is more efficient and produces more lift. The rear of the disc wants to descend, but because of gyroscopic precession, the result is that the helicopter wants to roll to the right. The transverse flow effect is also referred to as an Inflow Roll.
The transverse flow effect is felt as a vibration when just below effective translational lift (ETL) on takeoff and after losing ETL on landing.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-22
Principles of Helicopter Flight, 2nd Edition, pg. 101
FM 3-04.203-2007 Fundamentals of Flight pg. 1-42