Some of the people I ride with don't know what real hand signals are. They weren't riders before bikes had turn signals on them.
To tell people behind to slow down, you extend your left arm down, elbow held at 90 degrees, hand held flat, palm to the rear. No waving, no flapping. It means, stop or slow down, because I am.
If you want to tell someone coming the other way on a bike to slow down, the up and down flat hand is probably universal. But it can be misunderstood by people following you if riding in a group, so don't do it then.
Some of the people I ride with flap their arms like they are trying to fly to indicate they are going to slow down significantly. Most don't anymore, last meeting I explained they looked like doo-doo birds trying to take flight.
Even though I have signals, I still hand signal when riding in a group. I don't need some day dreamer running up on me.
Living the Gypsy Life