Are you talking about the spring that pushes down on the two hop arms?
The one to the right of this picture:
I personally like to take mine out becuase I like how the hopup acts without it. Your gun will still work without it. What that spring does is add more pressure on the hop arms so you can have a more constant and even hop setting.
The one to the right of this picture:
I personally like to take mine out becuase I like how the hopup acts without it. Your gun will still work without it. What that spring does is add more pressure on the hop arms so you can have a more constant and even hop setting.