T-Mobile has revealed its Uncarrier 8.0 announcement: it's called Data Stash. But a better name is probably data rollover. The fourth place US carrier will now let select postpaid customers carry unused data into future billing cycles. "High-speed data you don’t use each month automatically rolls into a personal Data Stash so you can use it when you need it for up to a year," the company said in a press release. To take advantage of the new promotion, you must be a Simple Choice customer who has "purchased additional 4G LTE data." T-Mobile says customers must be paying for at least 3GB of data each month for smartphones and 1 GB or more for tablets. Customers with a lower data ceiling won't be able to count on data stash to help avoid throttling.

Data stash is only for customers who buy extra data

The Data Stash program will kick off in January, and T-Mobile will be giving qualifying customers a 10GB pool of data to start off with. Once that's used up, T-Mobile will tally up your unused high-speed data each month (rounding up to the nearest megabyte) and add it to your data stash. The carrier says there's no limit to how much data can be stored up for future usage — so long as you burn through that data within 12 months. For instance, any data you stash in February of 2015 will need to be used up by the following February or it'll disappear. But Legere seems confident it's yet another idea that will pull customers away from Verizon and AT&T. "I think this is going to be something that's really going to shake up the industry," he said in an interview with Yahoo's David Pogue this morning

T-Mobile's previous Uncarrier announcements have included Music Freedom, which lets customers stream music without affecting their data usage, free overseas data coverage, and a Test Drive program that lets curious wireless shoppers test T-Mobile's network for a week.