It's not derived from FTP, it's derived from Heart Rate Variability and % of maximum HR at submaximal steady-state exercise sampled at the beginning of a workout. If you use the data field "performance condition" it will show you + or - your current status and your most recent HRV, you can use this figure on long rides, e.g. if it's plus 5 near the end, go hard, if it's -10 at the start, take it easy. It's similar to looking at what your HR is for a given power output sampled throughout the ride, but more accurate and sampled at periods that seem to try and eliminate artifacts and things like cardiac drift.