When used with a container stock, this tool makes a compaction-free cavity. It does this by cutting and removing an earth plug. Doing this gives a seedling's root system a better chance of surviving.
Dibble heads are especially useful in heavy soils, or sites compacted by machine. To use, place the head at the centre of the tool with a footstep. Various sizes are available to suit your needs. The handle, sold separately, is found here