Thatch is the organic matter that builds up between the soil and roots of lawn. A 1/2 inch or less of thatch is healthy for your your lawn. As this organic layer breaks down grass clippings, leaves, and other matter it helps feed your lawn. It also act as a mulch layer to help conserve moisture. Lawns naturally build up a thatch layer. Overtime the thatch layer can build up and if that layer gets to thick it prevents water, air, and nutrients to reach the soil and root system, and the lawn will suffer.
Power raking
Power raking is an aggressive process to remove thatch. It uses a machine to beat the thatch out of the lawn with spinning tines. The tines our set low, and as they spin they dig into the soil pulling up nearly the entire thatch layer. This process leaves the lawn nearly bare of needed organic nutrients and it is rarely the best option.
Dethatching
Dethatching is less aggressive and does not damage your lawn like power raking. This process is more like combing your lawn with spring tines which pulls up and loosens the thatch layer so that water, air, and other nutrients can enter the root system. The excess thatch is bagged by mowing behind the tines.
The simple answer is when the thatch layer needs thinning or loosening. However, the best time to dethatch is when your lawn is actively growing and the soil is moderately moist. Spring or fall is best for cool-season grasses. For warm-season grasses, late spring through early summer.