So what is TrainerRoad? Basically its a piece of software that you load onto your laptop and using an ANT+ cadence sensor (I'm using a Garmin GSC 10), ANT+ heartrate monitor belt and an ANT+ USB stick (again from Garmin) it works out your virtual power. That's not all though as there are workouts that you can download on-line that you can do and you get live heart rate and cadence displayed on screen as you work out. The workouts are based on power so by entering the turbo trainer you are using TrainerRoad uses the turbo's power curve to work out virtual power so you have to keep to the virtual power levels as instructed in the workout.
I appreciate that the power measurements are virtual but if you use the same tyre pressure and tension of the roller against the wheel then you'll get repeatable results as you have ironed out the variables. By completing a threshold test first, TrainerRoad will calculate your FTP (Functional Threshold Power) and use this for your workouts.
This does cost $10 a month but there are plenty of work outs and training plans to keep you busy and the boredom at bay.
Have a look at the video below if I haven't explained it very well.