For each feeder, a photo transistor scans its own track of an endless 35-mm film, giving a selection for each jack control spring as it passes the control position of the feeder.
This sentence has been recited from Knitting Technology by David J Spencer. Does giving a selection indicate an ongoing action?
A photo transistor scans its own track indicates a general fact, i.e it is in simple present tense. Sometimes phrases like giving a selection confuse me, because no auxiliary verbs are used before the ing-form of verbs, and such kind of phrases are set off by comma without their subject. Giving a selection starts with ing-form of verb, and no subject is directly attached with it, and it does not have any verb. Since no auxiliary verb is used, it is complicated for me to understand when such kind of phrases indicate an ongoing action and when they, instead of indicating an ongoing action, indicate the action that occurs before the other action.
I would request the experts of this site to discuss this problem. They may use my example if they consider it useful, or they may use other examples if they feel better with their own examples.