The only stupid question is the one not asked! 
From my “Debunking Exercise Myths: Part II:”
One of the most common misconceptions regarding belt use in a resistance training context is that simply because the belt assists in increasing intra-abdominal pressure (IAP), it must automatically reduce the compressive load on the spine. This is completely false; the belt certainly doesn’t have a favorable impact on compressive forces, and may even increase the compressive load! (5)
It’s readily apparent that wearing a belt has helped many lifters to move heavier weights than they would under “raw” conditions, but the question remains: do they decrease the risk of injury? Well, in consideration of the fact that they restrict the end of the range of motion in lumbar spine flexion, one would have to agree that they do, especially in those who cannot maintain a neutral spine.
Interestingly, this is one of the reasons belts can add pounds to your total; they facilitate the elastic response to torso flexion; the more neutral the spine is (as it should be), the less profound this effect is. Oddly enough, as McGill puts it, “to obtain the maximal effect from a belt, the lifter must lift poorly and in a way that exposed the back to a much higher risk of injury.”(5)
An additional mechanism by which belts increase one’s ability to move big weights are via expansion of the base of support to increase torso stiffness when placed under heavy loads. This stiffness helps to prevent the spine from buckling. (As someone with a pretty sound knowledge of biomechanics, I can assure you that buckling is a bad thing.) Belt use and “natural” methods to increase IAP are both effective in enhancing stability, both individually and in tandem.(8)
So what’s the problem with belt use? It alters firing patterns such that the belt becomes a crutch, and important core musculature is not called upon to stabilize the spine. Considerable evidence exists to suggest that wearing a belt causes individuals to unknowingly alter their motor patterns.
Cholewicki et al. (1999) compared belt use and increasing intra-abdominal pressure “naturally” under situations where lumbar spine stability was challenged via a sudden load release (in either trunk flexion, extension, or lateral flexion). The investigators found that belt use caused activity to decrease for the thoracic erector spinae in extension and the lumbar erector spinae in flexion (8).
[quote]trailer36 wrote:
EC,
This is a most basic question but i will go with it anyway. I never use a belt while lifting and the other night while i was doing dead lifts i used one that happened to be lying around the gym for my last heavy set. needless to say, i tried for 5 lbs over my PR and got it quite easily. my question is how exactly does a belt help assist a pull? i never would have thought it would have helped at all but low and behold, new PR!!![/quote]