Thatch is a tightly woven layer of organic matter that sits on the interface of the soil and the grass blades in the lawn. It should be removed when it gets too thick because insects and diseases may be harbored in this layer. It also inhibits the proper growth and development of the turf.
For cool-season grasses, power-raking should be done between late summer and early fall. For warmer locations that have warm season grasses such as zoysia or Bermuda lawns, mid-June is the appropriate time to power-rake the lawn. It should be done when the lawn is actively growing and has time to recover from the stress that the power-raking will cause.
Power-rakes work by rotating their thick, steel tines perpendicular to the ground surface, chewing up the thatch. The depth should be set so that it penetrates the layer of thatch fully and disturbs the very top of the soil surface. Cover the entire lawn area in a criss-cross pattern, making sure to get the thatch from all angles to entirely dislodge it.