The other version is a mechanical impact wrench. Activated by repeatedly yanking on the rope with full body strength. Both versions of the tool get fitted onto the fuze first like in the photo.
One detail I love in the later Ender's Game books involves teleporting a properly apocalyptic bomb into a very small room with the people who launched it. One of them pops it open and basically clicks Disarm > Are You Sure? > Yes.
I think it's Ender himself who asks '... is that all?' The guy's like 'Oh, yeah. We made it very easy to turn off.'