I've been through here on and off myself a bit these days, so thanks for getting back to us, at any rate.
To my understanding, Hung Kyun's Sui Long Paau Chui, or "Wave Casting Punch", is named to evoke the buoyant, cyclic, and driving action of ocean waves, and has to do with moving the opponent's body-mass through the bridge.
To contrast it with the Pok Yik Sau/ "Wing Flap" technique, I learned two main variations of it. The first, as a fast snap such as you describe, similar to what a bird might do when startled, to keep another bird or animal at range.
The second way is executed with a full-arc, follow-through delivery. In Hung Kyun, this is a variant of Kau Chui, or "Clearing Strike". Lol, some might not call this a "Wing-Flap" at all, as I doubt we've seen a bird move quite like this!
