"The active future is *YAQU:M ... The Jussive originally had the short u
(reduced state), eg. *YAQUM which usually became YAQOM..."
From this I would gather that in Pre-exilic Hebrew
YAQU:M (ie short a and long u) = "he will stand", and
YAQUM (ie short a and short u) = "let him stand"
In Tiberian Hebrew this would be expected to become
YAQU:M written YQWM(where a=qamatz) = "he will stand", and
YAQOM written YQM(where a=qamatz)= "let him stand"