I would bet (with no actual evidence) that grappling styles are more likely to use the slap. Based on my experience the slap is more of a way to just get your hands on the opponent in a way that you can get a grip to perform a lock/throw. In a striking style, you want to hurt them as much as possible with it, or have it move them to a compromising position.NGA, and Daito-ryu (from which it primarily descended) are my easiest examples. I'm not aware of that kind of heavy hand conditioning ever being a part of Daito-ryu, and it hasn't ever been for NGA (which may or may not be "traditional", depending which definition we're using).