transverse flow effect or in flow roll happens when just starting off from a hover to foward flight.
when hovering, there is a column of air coming down through the rotors and u use power to hover in the down draught.
as you move foward, the front of the disc starts cuting clean air (air that hasnt been effected by the rotors yet) it gets more lift because the induced air flow is recuced (downward flow). so now the front is getting more lift than the back so the blade will fly up as it turns to the left (on robbo) producing a high blade on the left. this makes the helicopter roll right unless the pilot corrects it with left cyclic. and because the induced air flow is less the helicopter with start to lift up with the extra lift, so you can push it over to get some more speed instead.
