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
The flash was a Mamiyalite ZE, a simple unit with fast response time.