Etymology of Old French 'escorgier': How does 'bind' evolve to mean 'whip'?

Question

scourge (n.)
c. 1200, "a whip, lash," from Anglo-French escorge, back-formation from Old French escorgier "to whip," from Vulgar Latin excorrigiare, from Latin ex- "out, off" (see ex-) + corrigia "thong, shoelace," in this case "whip," probably from a Gaulish word related to Old Irish cuimrech "fetter," from *PIE root reig- "to bind" (see rig (v.)). ...

I heed the etymological fallacy, but what are some right ways of interpreting the significance of the PIE root in this etymology, to make it feel reasonable and intuitive?. In particular,

1. How did *reig- "to bind" evolve to mean whip (I refer to the one greyed)?

2. What happened to the prefix ex- "out, off"? It doesn't look as if though it evolved, because the italicised states only to whip, NOT to whip OUT or OFF, whatever this latter means.

Answer 1

The sense "bind" gives a noun for a thing you bind with, i.e. a "thong" or "strap"; this in turn comes to mean "whip" because a whip and a thong are similar things; and on this is based a verb meaning "to whip". The prefix ex- in Latin doesn't always literally refer to "motion out" -- in some compounds, as presumably here, it can mean something like "thoroughly".