Due to the action of lots of foot traffic, the top inch of soil becomes capped and compacted, particularly in clay soils. This forms a semi impervious layer stopping water getting though to the sub soil and underlying drainage.
Deep slitting can alleviate surface compaction, by allowing the water to drain into the sub soil or underlying drainage. Slitting causes very little surface disruption, allowing pitches to be played on almost immediately after completing the operation.
Verti-draining although more expensive does a more vigorous job. Tines up to 1 inch in diameter are pushed into the ground up to a foot in depth, heaved slightly and pulled out. This has the effect of actually lifting the soil and breaking up the compaction. It can be slightly disruptive to the surface on very compact areas.
Either of these operations should be done between January to April and September to November as surface conditions allow.