what is a flank drive socket?

I see 'flank drive socket' here and therewhat is it?

SNAP-ON 1/2 Drive 6-Point Metric 27 mm Flank Drive® Shallow

STARK USA 3/4 in. Jumbo Flank Drive Impact Socket Ratchet Set 6

Snap On 3/8” Drive 6-Point Metric 18mm Flank Drive Shallow Spark

Flank Drive Sockets

Snap-On® IMM302 Metric Shallow Flank Drive Impact Socket, 30 mm

US Patent: 3,273,430 - Wrench Sockets, Socket Drives and Similar

Snap-on YTMM13 Metric 1/4 Drive 13mm Flank Drive Socket new

NOS Snap-on 1/2 Drive 1-1/8 12 Pt. Part number SW361. Never used, but has been engraved. See photos. NSN 5120-01-335-0827.

Snap-on SW361 1/2 Flank Drive 1-1/8 12 Pt. Socket 5120-01-335-0827

SNAP-ON 1/4 Drive 6-Point SAE 1/2 Flank Drive Shallow Socket

16PCS 3/4 Dr. Flank Drive Socket Wrench Set

Snap-on® proudly introduces new, completely reimagined Flank Drive

Taparia 1/4 Square Drive Socket Flank Drive

Socket 1/2, Flank drive 14 mm - Proffsproduktershop