This is our experience with the upwind pitchpole:
1) You are zooming along, perhaps a bit too footed off.
2) Then a little lull comes.
3) Helm bears away a bit.
4) Boat heels a bit.
5) Crew eases main a bit.
6) You are zooming along, perhaps a bit too footed off.
7) Then a little lull comes.
Helm bears away a bit.
9) Boat heels a bit.
10) Crew eases main a bit.
..repeat..
Soon enough you are absolutely caning along sort of close-reaching, when a violent pitchpole is a real possibility.
At the heart of the matter is the helm and crew agreeing about how flat is flat. If you attach your water bottle athwartships to the transom bar, you can use is at a spirit level, which gives you both something to look at. The water bottle should have a reasonable size bubble of air in it, obviously. There is a photo of us is Atum in Carnac 2006 I think with this configuration and the bubble can be seen.