It can be a backhand, so long as you convey it powerfully. It's a specific type of unarmed attack. Just as the entry: Longsword +15/+10 can mean thrusts, slices, or anything else you can really do with a longsword for the 1d8+str damage. How you hit is all in the description, not the name of the attack.
Having lesser critters fly across a room, and collapse against the wall, dead before they hit... That's an excellent way to convey a slam attack. So is a hammerfist punch that barely misses and splinters the wood on the door. Slams convey POWER (which is why they get 1.5 strength, same as a 2 handed weapon). Treat them kinda like the James T. Kirk double-fisted hammer punch (patent pending).