The scene cut during an Ultra's transformation which generally depicts them as rapidly growing larger with their arm outstretched. Why this specific cut is known as a "gun gun cut" is not known, but the term has been used officially. Otherwise referred to as the 'Ultraman Rise' or 'Ultraman Rising' pose.
While most poses during the cut generally only have one arm outstretched, depending on the cut parodied, both arms can be outstretched, the hand can be in a number of poses (most commonly as a fist), and some later Ultras even have non-static poses during cuts. A select handful of Ultras do not raise their arms when transforming, whether in specific contexts or generally, such as Ultraseven using the Ultra Eye.
The most commonly parodied version of the cut is Ultraman's, with only one arm outstretched and his hand thrust into a fist.