The Shift page compares your heading with a reference heading for the tack you are on: a lift in green, a header in red, in degrees, with ten minutes of history under it.
References
The app learns each tack's reference by itself: 20 seconds after a tack it waits for the boat to settle, then averages your heading for 30 seconds. Until then the caption reads SETTLING and LEARNING.
To set one yourself, sail your best upwind heading on that tack and tap Stbd or Port. Hold Auto to forget both and learn them again.
What else it shows
- WIND is the wind direction worked out from your two references (WIND · REFS) or, before you have both, from a pinged line (WIND · LINE).
- DDW is the dead-downwind heading.
- TACK ANGLE is the angle between your two references.
- UPWIND VMG kn is your average speed made good upwind, learned over the last minute of upwind sailing. It is also what TTL on the race page uses.
Which heading
The phone's compass is used once the app trusts it: steady, and agreeing with your course over the ground when you are moving. Until then, and whenever it disagrees, the page works from COG and the caption says COG. A compass offset for a phone that does not sit square in its mount is set in Setup.