This brush deforms the topology of the mesh while minimizing changes to the geometrical shape of the mesh. By default it will drag geometry, but this can be changed in the Deformation settings.
This brush is especially useful for redistributing topology to areas that require more detail, or sliding geometry to somewhere where they should be.
Holding Shift changes the brush effect to Relax geometry, creating an even distribution of topology.
Slides the topology of the mesh in the direction of the stroke.