T-Mobile is announcing a new program today that replaces the popular T-Mobile Test Drive offer. Starting on September 12th, you can sign up for a JUMP On Demand plan and T-Mobile won't penalize you if you decide to leave. You can even keep the leased phone to use on other carriers if you continue making payments on it.
This is a viable replacement for Test Drive because you have the option to cancel in the first 30 days for a full refund. You get all service and device fees back and return the phone. After 30 days is when things get interesting. Should you decide to leave T-Mobile, you'll get the current month of service refunded and have the option of keeping your phone. These are leased devices (Jump On Demand), so you have to keep making payments on it, but T-Mobile will unlock it so you can use it on other carriers.
That's really the gist of the program, so I feel like "Lifetime Coverage Guarantee" is not a very accurate name. Even though Legere is talking up band 12 and coverage, it's not really anything about coverage. This is a satisfaction guarantee that applies to Jump On Demand lines. It's just easier to leave the carrier without incurring penalties. Rather than give back the leased phone or buy it outright, you just keep paying for it monthly.
The T-Mobile blog post is heavy on iPhone mentions, but the Lifetime Coverage Guarantee is also available on select phones from LG and Samsung. That probably means all the devices currently eligible for Jump On Demand.