Put more simply, a real slapstrap completely replaces the silicone strap that comes with your mask, the idea being that the silicone strap can tear apart and cause you to lose your mask. Either thin nylon webbing or long velcro tabs are sewn onto the neoprene pad and are slotted through the mask as a standard strap would.
There are also neoprene covers that look just like slap straps, but these are hollow sleeves that simply slip over the original silicone mask strap. The neoprene cover makes it easier to put the mask on and off since it doesn't catch against hair or hood, and may be easier for some to manage because it benefits from the elasticity of the underlying silicone strap, but it doesn't address the "silicone straps can tear" issue.