Jumpmaster mode questions

Just did my first dive in jumpmaster mode.

1 - press 'go to HARP' is to start the dive.

When do I press 'go to HARP'? In the plane right before I jump?

2 - on the way up, it went into power save mode. Do I need to stop that?

3 - after the dive, the watch was locked in dive mode. I needed to shutdown (press LIGHT) then press UP when I saw the Garmin logo, and answer YES to end the dive.

Why did it go into dive mode and how do I prevent that?