Scarifying is the removal of lateral grass growth and dead grass (thatch) which builds up leading to a distressed sward and deteriorating lawn.
A scarifier utilises a rotating cylinder cassette with knife tines that cut into the surface of the soil (or sometimes just brushing). It tears through all of the waste that builds up allowing air and light to get through.
Any heavy duty scarification is greatest carried out throughout early summer time (late April - early May) or late summer time (late August - early September). This is because growth should be strong and recovery is fast.