Following aeration or overseeding, a topdressing is applied to quicken healing or growth, as the case may be.
The topdressing consists of sand and fertilizer along with soil for aerified greens or seed for overseeded greens.
"The main purpose of topdressing is to improve drainage through the introduction of Horticultural/Sports Grade Sand to the roots. IT MUST BE Sports Grade Sand"
This helps to break down the thatch layer. DO NOT use loam or clay as the fine particles can undo the drainage effect you are trying to achieve."