From an US doc, not sure if 100% right.
Arming Function :
When the mine reaches the bottom, hydrostatic pressure starts the arming clock which may be set from
1/2 hour to 6 days. The clock starter may have a soluble washer, depending on the clock type used.
Starting of the clock also arms the antirecovery switches, if fitted. Also, the intermediate switch, if used,
electrically connects the PSE circuit or any other destruct circuits which may be fitted in the mine. When
the arming clock runs off its set time, it closes switches which activate the firing mechanism. The firing
mechanism then sets itself to the ambient magnetic field, and sets its own sensitivity relative to this
ambient field. The mine is then armed.