I switched from AT&T to T-Mobile about 2.5 years ago and they explained JOD to me so I decided to go with that as I like the option of being able to switch phones without expensive penalties. From what I remember I can switch 3 phones within one year, each time I make a Jump it costs me $99 ($0 for base model of device)... if I remember correctly. I haven't had to use it though as I've been a Samsung Note user, so I just upgraded once every year.
Since the Note 7 issue though, this may be the first time I take advantage of it. I currently have the S7 Edge, I'll grab the larger version of the S8 (if there is more than one version) when it comes out in Spring and then in Fall time if they come out with a Note variant that's a little larger with a stylus I'll jump to that.
Side note, with T-Mobile I got my unlimited data back, got mobile hotspot and got the binge-on that doesn't affect data as long as I'm using one of the approved video providers (also doesn't count against mobile hotspot to watch on my tablet as long as I use an app version of the video provider). I've been very happy with T-Mobile and haven't had any issues, wish I had switched earlier.