Each of the cells in a tray have large openings at the bottom, and trays are supported on racks in the nursery to enable air to flow under and around them. As the roots attempt to grow out of these openings, the air flow 'burns' the root off. This process is known as 'air pruning'. This encourages the plant to grow more roots inside the container without 'circling'. This produces a large amount of root material that has been trained to the bottom of each cell.
Some trays have slits in the cell walls to train and prune the lateral roots also.
Many white root tips are produced which are ready to grow vigorously when planted out. In fact, all of the available cell systems were developed for large scale planting. This saves time and effort in recovering pots from the planting site, and enables a large number of plants to be transported in one journey. Saltbush and native grasses grown in 'speedling' trays are an example. Note however that the smaller volume for roots with cell systems means that the margin for handling errors and fatalities from drying out is smaller than with larger individual pot systems.