Dayt Cob CL copy.jpg
This is the standard grip to have on the steel slapper, (from "metalmen" / craftsmen going back to 1919 - Frank Cufaude, Jim Taylor, Walt Borland, Gene Halvorson ...)
Hand is firm on handle, index finger extended onto back of striking face, for control/accuracy. The slapper will acquire a shiny spot from that steady finger.
Alternate grip, for a different body position - when your body is lower than the panel.
Dayt Koop CL copy.jpg
Whackity-bambambam until the HAZ is level, distortion and stress gone, softness back to hard, and weld bead tamed enough for the file.
Thus:
Dayt coop CL copy.jpg
and
RSK copy.jpg
etcetera