I'll let the ad below from their website, explain how it works. It only works on trainers (a pretty wide variety of trainer and roller models), and you have to have your computer standing by with the software (which won't be free forever), but I'll tell you what: For $0 the price is right as a supplemental power meter through the winter until I get a powertap, and even then I'll take this display while I'm indoors over my little Garmin any day! Thank you TrainerRoad.com - you guys are awesome!
Training with power on a budget!
I would first like to point you to my set-up for riding indoors - true scrappiness. Notice that this professional draft-legal racer is still riding an aluminum road bike. Notice there is a ratty old towel where there should be a nice rubber trainer mat, and a jerry-rigged riser block made from an old Harry Potter book and some magic markers (to keep it from sliding out from under the wheel). Even that nice trainer was a second hand sweet deal from my work hook-up at Landry's Bicycles. But wait, what's that off in the distance on my computer screen? Looks like a fancy timer. Well, I'm glad you asked - that is a new application in beta, produced by TrainerRoad.com. Get this. It uses the power curve for my specific model trainer (the variable coefficient for resistance - not that this terminology is any easier to understand...) from my trainer combined with my wheel speed to calculate my power output. That's right baby, I'm training with power!