The distinction between out of trim and side flight is an important one.
Side flight is tested and approved, follow the flight manual. It is not "out of trim" and is not dangerous.
Forward flight out of trim is what is the difficulty, where the pilot uses large pedal inputs while in forward flight.
Out of trim is a problem for teetering rotor helos because the horizontal tail and fuselage create fairly large aerodynamic inputs that try to upset the aircraft, so the pilot puts in large cyclic control inputs to keep the aircraft level. These cyclic inputs create large flapping situations, perhaps close to mast bumping. If the pilot is out of trim and also maneuvers the cyclic a bit, it could be even worse.
These tips are for all teetering rotor helos, even Hueys and Jet Rangers. In fact, the sideslipping (out of trim) issue was uncovered during US Army tests of Hueys after a mast bumping accident.