Observations of low-level vorticity couplets within RFDs indicate that tilting of vorticity by the RFD is important in the formation of tornadoes within supercell storms. During the tornadogenesis phase in supercells, the parcels of air infiltrating the tornado or incipient tornado regularly seem to pass through the hook echo and RFD, which can serve as the basis for Fujita's "recycling hypothesis". Furthermore, observations of the clear slot during and just prior to the tornadic stage, imply the air infiltrating the tornado may come from the RFD.