Over time your lawn builds up a layer of thatch which act as a barrier to water and air penetration into soil. the concept of the de-thatcher is similar to an aggressive rake with a hard back garden rake taking the layer of thatch up off the soil. Don’t be alarmed by the appearance as the lawn will fill back in as the nutrients are able to sink in. You will find this to work best if you wait until your grass has greened up after winter and can be done at any point after that providing you supply enough moisture to help it recover.