Picture related. Shaking comes from the initiation of the fight or flight response due to excessive anxiety. Adrenaline is released, but due to there being no "real" threat the excess epinephrine just makes you shake. This is one of the definitive signs of general anxiety or PTSD, if this happens when you're not in actual danger. The entire gun enthusiasts movement essentially co-opted being triggered, which is a thing that actually happens to people with PTSD when exposed to stimuli associated with their traumatic experience. When a gun enthusiast says they're "literally shaking right now" they're saying they were triggered and its manifesting in symptoms similar to anxiety and PTSD. They're basically pretending to have a mental illness to get what they want. This is called Amygdala Hijack.