SNES Asterix & Obelix What graphic-technic of the system makes the waterwaves wiggle?

[Drachenbauer]

In Level 2 of the SNES-game "Asterix & Obelix" there are huge water-waves to pass by boat.
They have a really cool effect for the low graphics-system of this nostalgic consol.
You can see it clearest, when you pause the game.
The shape of the water waves wiggles a bit.
Not only the main layer, that supports the boat, but also the parallax-layers in foreground and background.
You can see it here from 5:40
Gameplay-Video
How is this effect made?

[Raccoon Sam]

Using BSNES+, we can see that it's a basic setup of 3 layers; two 4bpp foregrounds and one 2bpp background. Nothing out of the ordinary here, many games do this.
Asterix & Obelix however, combines not only parallax scrolling (moving horizontal slices of layers mid-frame) with this, but also priority switching mid-frame so that parts of layer 3 are in the background at the frame's top segment but also in the foreground at the frame's bottom segment. Neat trick, very similar to that which Yoshi's Island does sometimes.