"Stomach pumping" is an unfortunate choice of words to describe the process. You're not "pumping" and it doesn't really involve the stomach. First, a little water is drawn into the tip of the instrument, then it is inserted into the trout's throat. When the water is expelled into the trout's gullet the contents are washed up and out. Properly done, it's not a particularly invasive process, gripping the fish tightly enough to hold it still while carrying out the action can potentially be more harmful.