Posted By:
brian parkerThe handful of slabbed cards that I have owned and cracked (in my hobby dictionary slabbed = commodity), I used a bench vise to secure the slab, with the top of the jaw lined up right along the ridge beneath the label (I have only cracked PSA cards). Then by applying equal pressure along the rest of the holder I pull it back towards me and snap off the top. A slotted screwdriver does the rest of the job.
Still, I would probably advise TBOB's method, as he has a lot more experience in these matters. I have provided this information just in case you are in the middle of the desert and need to crack a holder, and you only have a bench mounted vise available.
Brian