GR translation: hep to the jive

[Mike Jing]

P179.29-39  It has begun to reveal itself: how easily she might go.
For the first time he understands why this is the same as mortality,
and why he will cry when she leaves. He is learning to recognize the
times when nothing really holds her but his skinny, 20-pushup arms. .
. . If she leaves, then it ceases to matter how the rockets fall. But
the coincidence of maps, girls, and rocketfalls has entered him
silently, silent as ice, and Quisling molecules have shifted in
latticelike ways to freeze him. If he could be with her more . . . if
it happened when they were together— in another time that might have
sounded romantic, but in a culture of death, certain situations are
just more hep to the jive than others—but they’re apart so much. . . .


"if it happened when they were together—"  If what happened?

And what exactly is the meaning of "hep to the jive" here?