GR translation: her typewriter banking in a phalanx of letters between them

[Mike Jing]

V708.40-709.2, P723.1-4   What Jessica said—hair much shorter, wearing a
darker mouth of different outline, harder lipstick, her typewriter banking
in a phalanx of letters between them—was: “We’re going to be married. We’re
trying very hard to have a baby.”

Is "banking" a present participle modifying "typewriter"? If so, what does
the verb "bank" mean here?