It's only the previous month of unused data from that month's allotment (and that data gets used first), but that's better than nothing, I guess.<p>Assuming 15GB plan:<p>Month 1 (15, 0 avail): Use 10 GB, roll forward 5<p>Month 2 (15, 5 avail): Use 8, roll 7, lose the previously rolled 5<p>Month 3 (15, 7 avail): Use 20, roll nothing (lose the unused 2)<p>Month 4 (15, 0 avail), etc. etc.<p>So this saves you if you have one outlier month, but it's still not as good as rolling what I've nominally paid for.