What is a "zip-zap"?
Solution 1:
The picture isn't what is generally known as a zip-zap. The picture shows a Catherine Wheel.
A zip-zap comprises a number of projectiles which are launched in sequence, in a fan-shape from left to right and back again. Here's one of 35 shots, and video of it in action.
I guess the name comes from the reciprocating nature of the display.
Video of zip-zap in action