I've never seen the hip slap either. Our group uses the "after you" signal.
Oh man, this.ess experienced riders signal too much. More experienced riders understand what is going on and require far fewer 'signals'.
I am sometimes amused when I ride with a 'club' (social ride) at how much chatter goes on and how many hand signals I see. It is useful for the group but more info than I need. I actually have to check myself and pass on signals I wouldn't normally bother with because I know riders behind me are expecting it. This varies from group to group and is often the most stressful part of the ride for me.
My usual group only signals for stuff that is life-threatening or has changed from the usual. Roadkill, for example, gets a point because it probably wasn't there last time. Big potholes will get a point. But that gravelly spot along Snake road? We all know it is there. No need to point it out.
I rode with a group in Fredericton who pointed everything. Leaf on road? Gum wrapper? Tree shadow? Point point point. The whole ride was "lookatthat!" Drove me crazy because I was condioned to treat points as being very serious, so the non stop pointing made me jumpy. And on my pulls, when I didn't point, the rest of the ride got cranky with me. They were a great group and very friendly, but the ride wound up being very stressful.
But nowhere NEAR as stressful as the Toronto Donut Ride, where nobody signals anything. 75 km of sauve qui peut. My SA during that ride sucked and it felt like we were always a heartbeat away from disaster.