I think mostly horizontal play....but you know it works to help increase retention as well...specially with AZwelke making it.

This whole concept has me thinking about using the Scrapyard sheaths in different ways. I'll post a picture in a minute of the Regulator Sheath with the DSSF installed...it's a great fit, except the liner is too big (similar issue to the Combat Master).

I could see this on smaller knives too....take the S6 sheath from Scrapyard, and mold Kydex to fit the Boss Jack, for example, or reduce length for smaller knives like the Dumpster Mutt, Infi-Coot, etc.