From J Wiki
u"v m"v Copy Rank Conjunction
The benefit of being able to use v itself, instead of its list of ranks, n, is: it saves you having to find the actual ranks of v if you want to create a verb out of verb u (or noun m) having the identical ranks of v .
a not-uncommon requirement.
v=: < NB. sample v - Box (<) v b. 0 NB. the list of ranks of v _ 0 0