You can search this blog and find the whole story but in short it's a one-wire, one-external-sensor black box (it's actually white) that uses variables like velocity, acceleration and altitude change coupled with your own choice of constants (weight, friction and aerodynamic co-efficients) to calculate power. It dumps all of that data into a reader and you can play with the .CSV file in any spreadsheet.
- The pros are that it's the cheapest option for constant (ie not just hills) power measurement and it's as easy to mount as a speedo (making it easy to swap bikes, too).
- The cons are that the constants aren't really constant. Your weight changes as you sweat and eat (but not by much, unless you are riding 200km/day and not refueling); your aerodynamics change as you sit up or crouch down (but you can adjust your aerodynamic constant to be a 'best fit' for your needs), or just sit in a draft (and yes, it does give some strange results when drafting big bunches or trucks). And your friction changes according to tyre pressure and road surface (but again, not by much).
- Batteries... buy a few CR2032's for spares... they can last a 2 months or 2 weeks, depending upon sample rate and time on each ride
- Some batteries drop below the recommended 2.75V on ride 2 - but bounce back on ride 3! So don't chuck it away too smartly
- But be careful, too, as you risk losing all ride data and probably getting spurious results with a low battery, but...
- I've had no obvious problem starting a 1-2 hour ride with voltages as low as 2.66V.. and it even ended the ride back at 2.72V