Actually ive been looking into the smartparts impulse and recently read the owners manual, near the beggining there is an entire section on it.
And the quoting begins (I have edited out parts that do not have anything to do with the reasons it gives. For the manual and its full readings please go to
http://www.smartparts.com/Manuals/Impulse.pdf)
"It is recommended that you do not dry-fire (fire without paint) your Impulse....Because of its low pressure design, the marker requires a paintball in front of the bolt to produce back-pressure, which makes the marker re-cock.....If the Impulse is fired without anything in front of the bolt, the bolt will remain forward
and allow air to continuously drain down the barrel."
So basically the manual advises against because of air leakage But it also says
"If you must dry-fire the Impulse, remove the barrel and place a rag over the front portion of the marker where the barrel screws into. This will simulate the paintball and create the required back-pressure to re-cock the marker."
Hope this helps