These pictures are scenes frozen by a flash unit that is triggered by the action being recorded. They were made by setting up a shot, opening the camera shutter using its "bulb" setting, and carrying out some action that opened or closed a circuit to trigger the flash.
The balloon pictures were taken by triggering the flash when the balloon began to shrink after being popped. In most cases the balloon had a strip of foil attached to it with probes pressed against it on two sides, with a circuit interrupted when the foil pulled away from the probes.
The firecracker pictures used pieces of small wire or solder wrapped around the firecracker, with the strobe being triggered when the wire was broken by the explosion.
The flash was a Mamiyalite ZE, a simple unit with fast response time.