Minimal spoiler version: Guy in the back (note he's tied up) is being used as a human hostage by the pilot here. (i.e. blow me up and your buddy gets it too)

But the pilot later releases him on his own accord, giving up the 'advantage'.

This is Ishiyumi-sensei's joke take on what might have actually happened. Especially since the "hostage" wasn't strapped in or anything and would have likely bounced all over the place in the cockpit like how it is depicted here.

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Anyway, currently tagging this spoilers because the movie is still new (though the novel and other adaptations have been around for ages). Spoilers should be removed later once enough people have been assumed to have watched the movie.