Spunk - if the helicopter yaws when you don't want it to you apply pedal to stop the yaw. If you reach the pedal stop and the aircraft is still yawing you have reached the limit of your tail rotor authority (max pitch). In this condition the torque produced by the engine driving the rotor cannot be balanced by the anti-torque force from the TR.
One of 2 things can happen now - either a. the rate of yaw will gradually decrease and stop, possibly because you have yawed back into wind and weathercock effect on the fuselage/vertical satbiliser has produced sufficient anti-torque force to balance MR torque, or b, the rate of yaw will increase and your only course of action is to reduce the MR torque (lower the lever) and/or gain speed (the TR benefits massively from translational lift).
The question you have to ask now is why?
Have you put the aircraft in a position (high AUM, high DA) where you are at the limits of the RFM and been caught out by slightly different conditions ie wind direction and strength than you expected?
Have you flown the aircraft badly and failed to anticipate the requirement for extra pedal as you lose translational lift (requiring both more torque to hold height and more TR thrust to maintain heading)?
Have you allowed a rate of yaw to build up because you weren't paying attention and the available pedal isn't enough to stop it?
Have you pulled so much lever that the Nr is drooping so that the TR is slowing down as well and reducing the thrust it can produce?
All of these could be described as LTE because it isn't doing what you want it to and there is not enough TR thrust. BUT they are all equally a case of NETR for the same reasons.
Ideally a helicopter will always have a surplus of TR thrust available to cope with all situations but if the TR is too small then it is much easier to encounter uncorrectable yaw in all the above situations - THIS is what critics claim is wrong with the 206, it just doesn't have sufficient surplus TR thrust to make the pilot's job easier.
It is thoeretically possible to put a TR into vortex ring state but I think the rates of yaw/crosswind would be phenomenal unless the aircraft was hovering at very low power and subsequently TR pitch (therefore the flow through the TR would be minimal and small rates of yaw could give an opposing airflow) ie in a strong updraught.
Sorry Nick you must have posted while I was writing!