In this game everyone could only jump once at a time, but by a another techniuqe we could double jump to higher place where zeds couldn't reach you.

prepare a boomstick


now with the boomstick in hand, this weapon alt fire(mid mouse) would provide a boosted kick, so now when you jump and alt fire at ground, it would boosted you to higher ground, so you could go to some high ground that Zeds couldn't reach.