Curious about something I saw at the gym

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I am new to going to the gym and pretty novice when it comes to weight lifting so I'm hoping someone can explain something to me. Tonight while at the gym I saw several people (men and women) using very light weights to do these kind of "twitchy" lifts? I'm not quite sure how to explain it. They were doing very short, quick versions of exercises I'm familiar with like bicep curls, fly 's and overhead presses. They would only move the weight an inch or two and fairly fast and they didn't do very many.

Like I said, I'm new, so could someone tell me what this was about? Is there some benefit to this that I'm missing out on? I felt kind of self conscious curling 20 pound dumbells next to a guy doing those twitchy things with 5 pounders.

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