Scarifying
During the growing season a lawn will produce a thatch of dead and decaying material at ground level. A build up of thatch and moss will prevent water and nutrients from penetrating to the soil and grass roots. Scarification is the mechanical means of removing this thatch, allowing the grass to grow stronger and healthier.
When scarifying your lawn you should always start out so the blades are brushing through the grass and not touching the soil. Setting it too deep too soon might not gain you any benefits and will put needless strain on the machine. When you have done one pass of your lawn start again but go in a different direction i.e. diagonally across your lawn. Once you have done this if you believe there is still an excess of thatch in your lawn, lower the scarifier a few millimetres so the blades are now just brushing the soil and undertake the process again.