The idea is that taking an enzyme derived from beans that blocks the body's ability to metabolize starches into sugars will lead to lower glycemic load. You can eat your cake, and not worry about spiking insulin too. But does it work? Maybe. They're sold as supplements, unregulated and don't require testing to get on the market. Thus there haven't been a whole lot of trials done. The small limited studies that have been done are overall inconclusive.

Which leads to the second question -- are they safe? Well, if they do in fact block the digestion of starches then that means raw, undigested starch winds up in your large intestine where it ferments, produces gas, bloating, diarrhea, and possibly pain. I don't know of anyone exploding from using starch blockers and eating a high carb diet, but intestinal discomfort, the runs and stinky farts are not generally anything anyone welcomes with open arms -- or your partner would be happy about in bed.

If you really want to help yourself eat less carbs, eat more fiber instead. You'll feel fuller faster, for longer, and consume less overall carbs as a part of your meal. Why pay for the promise of magic pills when you can pay for yummy broccoli or asparagus instead?