As far as I know the reducer threads on like a silencer, but uses a rubber band to add drag to the BB and slow it down.
Negatives of this like stated is that if it does break the rubber band you'd be firing VERY hot for a CQB gun. And secondly it completely nulls the effects of the hop up because it would stop the spinning of the BB.
For the cost of one, just get a spring swap and save the worry.
EDIT- I type too slow, Pusangani answered it..