here is an explanation video with animations at around the 2:30 and 5:00 min mark.
The fast wind was moving more or less straight. Now that it slows down, it gets a wavy pattern, that also moves slower. As a result we get more stable cold zones and more stable heat zones. As these move slower over the planet, it results in longer cold and heat waves, with adverse weather. So the boundary doesn’t get “breached” in one direction, but rather it relaxes in both directions, getting the yo-yo weather you describe.
https://www.dw.com/en/when-the-jet-stream-weakens/video-62097932
here is an explanation video with animations at around the 2:30 and 5:00 min mark.
The fast wind was moving more or less straight. Now that it slows down, it gets a wavy pattern, that also moves slower. As a result we get more stable cold zones and more stable heat zones. As these move slower over the planet, it results in longer cold and heat waves, with adverse weather. So the boundary doesn’t get “breached” in one direction, but rather it relaxes in both directions, getting the yo-yo weather you describe.
Thanks, that makes it a lot clearer for me.