You are most likely to encounter it during a low or zero forward speed descent at a medium rate (500-1500 fpm) and a high power setting, typically found in a steep approach, where the column of air remains underneath your helicopter. To get out of it, reduce power, enter forward flight or autorotation, but bear in mind that you will lose a lot of height anyway. Better still, keep out of it with positive forward speed or by descending more gently, so the doughnut is below the machine.

Specialised Tasks

Some of the more exotic things you can do with helicopters include bombing avalanches, rapelling (that is, dropping off people to fight forest fires, otherwise known as dope-on-a-rope), wildlife capture, aerial ignition, water sampling, where you hover very low over a body of water and a scientist dips the equivalent of a jamjar into it (I still haven't found out why they don't just use a boat), or frost control, where a large barrel of oil is lit to provide smoke that will indicate the level of an inversion. You then fly with your rotors just above the smoke to bring the warm air down and prevent frost on crops.
Note: Some of this is dangerous! Don’t try it without training!

External Slung Loads
A helicopter can go where cranes are impractical or more expensive, or you might not be able to get a load inside the machine (maybe you wouldn’t want it there anyway, if it’s a dead animal or explosives), so you try and lift it.

In theory, you can lift anything, provided the payload is available; I've even been asked to quote for lowering 800 feet of unrolled telephone cable down a mine shaft, because the drum it was rolled on wouldn’t take the weight. However, more common tasks are logging, placing air conditioning or ventilation equipment on the roofs of tall buildings, pulling cows out of bogs, picking up water to put out forest fires (water bucketing), dropping solution over forests (top dressing) or moving seismic equipment about. In fact, many tasks done with a helicopter are really extensions of load slinging, and, in remote areas, this will be a major part of your bread and butter – really specialised stuff will be found as subheadings below. A typical length is 25 feet, but can be up to 200, so don’t forget to include the line as part of the payload – it will be heavy (try about a pound a foot).

If you get involved with the air-conditioning-type loads on top of buildings, you will need a permit from the local authority, who will also need a lot of advanced notice. Check also for any bye-laws, the local environmental people, police and fire services (who may charge you for extra crews).
Logging, officially, is removing felled and bucked logs from areas where all trees have been felled. It is very fast, with lots happening at once, and there will be a smaller helicopter to carry the used chokers every 75 minutes or so (chokers are lanyards with the equivalent of a slip knot which tightens as the load is taken up, making it more secure). It is not an operation based on finesse, as the machines are continually using full power cycles and undergo a lot of twisting, etc. If you’re planning to buy a helicopter that has been used on logging, inspect it very carefully!