It is a "single sweep" mode, useful for capturing (on film) a fast, isolated, trigger event. You set up the trigger threshold and wait for the event to occur with the camera shutter open. When the 'scope triggers a single horizontal sweep is produced and (usually) a "ready" light goes out to indicate the sweep has occurred. Then you close the camera shutter and process the film. Useful if you can find a camera that fits and you can find instant Polaroid film packs. It was all the rage (and the only way) to capture transient events before analog storage... and later digital storage... 'scopes came along. The analog storage tubes were finacky to operate and very expensive. I had one in my lab but never used it in that mode. I had to buy my own DSO after I retired.
If you can control the occurrence of single events (like with a command to your microprocessor) you want to capture, and don't mind relying on your memory, you can turn the lights off in the room, let your eyes get dark-accommodated, and observe the single trace with your Mark I eyeball. Or do the same thing with an ordinary digital camera if you can open the shutter, fire off the event, and close the shutter. Messy and prone to error. Best to buy a DSO.
