Sprint's been taking aggressive steps to reverse its fortunes under new CEO Marcelo Claure, and today the carrier is making a bold bid to steal customers away from Verizon Wireless and AT&T. Sprint's new "Cut Your Bill in Half" event promises that existing customers of the biggest two US carriers will pay half their normal billing rate by making the switch to Sprint.

The limited-time promotion kicks off Friday, when potential switchers will be able to bring an AT&T or Verizon bill to any of Sprint's stores and see if the savings are worth making the jump. It sounds simple enough; on Friday, you upload a copy of your current bill to Sprint's website (or take it to a store). From there, Sprint has pledged to cut a Verizon or AT&T customer's existing rate plan in half. Under Sprint, your plan will include unlimited calling and texts, with the carrier promising to find a data tier that "most closely matches the data allowance in your current monthly rate plan."

But as is normally the case with these specials, there's some important fine print to be aware of. First, say goodbye to on-contract pricing. Your new phone must be purchased through Sprint's leasing program, Easy Pay installment billing, or bought outright. Second, you must trade in your current AT&T or Verizon smartphone when moving over to Sprint. That point is non-negotiable, and if you fail to turn in a phone within 30 days, you'll get slammed with a $200 "non-return phone charge." Ouch. And if you're porting a family plan, phones must be provided for each line that's coming over. Keep in mind you're not getting any money or gift cards for giving Sprint these phones; the requirement at least partially explains how Sprint is able to undercut its rivals.

Before switching, you'll also want to make sure the data plan you're on right now will be a good fit long into the future. If you ever need to jump up to a higher data tier, you'll lose the cheaper plan. Sprint says:

Because the Half Price plans are uniquely calculated based upon your current Verizon/AT&T talk, text and data plan MRCs, customers wanting to make plan changes will be subject to national rate plan pricing that is available in market at that time.

Once the rate's locked in, you don't need to worry about losing it. An FAQ page goes into other specifics and potential restrictions. Sprint still says it'll buy out your current contract, paying up to $350 to cover early termination fees. But that offer has its own, separate set of requirements. Sprint cautions that "new plan features may not be an exact match," so support for things like tethering remains up in the air.

Still, Claure seems to think it'll be a hit with wireless customers on the big two carriers. "The Cut Your Bill in Half Event clearly demonstrates that Sprint delivers the best value in wireless," he said. Notably, Sprint isn't going after John Legere's T-Mobile with the new effort; AT&T and Verizon are much bigger targets, and the Uncarrier's current rates may be too low for Sprint to halve.