Not a meteorologist, but this is the best way it's been explained to me.

Think of the jet stream as a river. When the equatorial air masses are significantly warmer than arctic air masses, the jet stream went like a river between two steep mountains, straight and fast, west to east. Cold stays north, warm stays south.

Now that the arctic has significantly warmed, the 'riverbanks' aren't as steep, and the jet stream slowed down and started to meander, like a river when it comes to a broad valley. Like a sine wave. So instead of having a clear line between warm and cold, we have a bunch of troughs and valleys. As these troughs and valleys move west to east we get a cycle of warm/cold/warm/cold/.

In the cold arctic scenario, it takes a lot more energy to move the jet stream north and south, taking days or weeks to move 200 miles. Now it happens in hours on a regular basis.

Something about the jet stream I see more often in the area forecast discussions is the jet stream getting low enough to clip the mountain tops. I don't know for sure if this is a new phenomenon or I just wasn't paying attention. Would be grateful for someone with more knowledge on to chime in on both parts.