What is the “Slim down old trips” option for?

For keeping an unlimited history without your phone or your backups growing without end. A trip recorded in Detailed mode is several hundred GPS points. After a few months of automatic recording, those points make up most of the space Fenzly uses and of the size of every backup, while most of them no longer serve any purpose: a track draws just as well with ten times fewer points.

The option, on by default in Settings → History, therefore does this:

• It only touches trips older than 30 days. A recent outing keeps every point, leaving time to review it, correct it or export it as GPX at full resolution.
• It freezes the figures first. Distance, duration, average and maximum speed, elevation are computed and stored on the trip before any simplification: what you then read on the card or in the history does not change.
• It then simplifies the track, keeping only the points that matter for the shape of the route, to within 5 metres, plus the start, the finish and the top-speed point. On screen, the track looks the same.

The result is storage five to ten times lighter for old trips, faster backups, and a history that stays complete as far back as you go. Turn the option off if you want to keep every point of every trip, for instance to re-export old outings as GPX in full detail; the price is heavier storage and backups. The depth of the history is set separately, with the retention duration (guide Reading my history).