Before takeoff I believe it is set by hand to set the takeoff trim.
The trim wheels always have priority over the electrical trim. Moving them in-flight:
- Bypasses the flight computers (an override mechanism with micro-switches).
- Disconnects the autopilot when moved beyond a certain threshold.
- Freezes the automatic trim.
The flight computers for normal stabilizer control (ELACs) remain active though, no change in laws would take place. They remain active to ensure they're synchronized with the manual action. Let go of the trim wheels, use the stick again in pitch, and auto trim will resume.
If you use the stick, while holding tight on the wheels, the flight computers will think the stabilizer is jammed, and the warning
STABILIZER JAM will show, with it the associated procedure.
During flight it is not touched unless the aircraft goes into direct law or "worse".
There are two scenarios where you'd need them that are better (not worse) than direct law. That is the aforementioned [actual] stabilizer jam, or
F/CTL STABILIZER JAM. The latter is when the flight computers lose contact (electrical signal) with the stabilizer (this scenario leads to alternate law with protections lost).
In both scenarios, if the pilot is able to move the trim wheels, they'll use the elevator position shown on the flight controls display to make trim inputs such that the elevator moves to the neutral position.
Source: A320 FCOM (don't use for flight).