Martin,
Here is a link to a DVD that I bought when I first bought hiking poles.
It is very comprehensive.
http://www.rei.com/product/751284
Per the video, you want to set the 2nd section, which is closest to the top of your pole/handle about "open/wide" the widith of your fist.
Then adjust the lowerest section so that your forearms are 90 degrees or about the height of your waist/hips.
All future adjustments will be made using the 2nd or upper section because it is closer to your hand and therefore easier to reach/adjust.
On the down hill sections, you will lengthen the 2nd section appropriately so that when you are swinging your poles on the down hill, they are long enough to reach out and down so that you are not having to bend forward or over extend your arms reach to have the tips plant on the ground comfortably to easy your pace and weight. Ultimately, you are looking to decrease the impact on your knees and body.
On the uphill sections of the trail, you will shorten the length of the 2nd section so that as you swing your poles up hill, they are planting easily and are not too long such that they hit the upward slope of the trail and prevents your arms from easily moving/swinging your poles.
The key is that as you are going uphill, you want to gently push off with your arms to assist you as you are traveling up a trail to take just a little bit of strain off your body.
As you are goind downhill, you are using the extended poles to control your pace down hill, protection your knees from excessive impact, and to use the tips of the poles to preventing slipping, falling, stumbling.
Hope that this helps.
The DVD is listed at REI at $15.00 and is a relatively cheap investment in how to effectively use your poles.
I honestly say that I could not have completed any of my longer trips without them.
-Tony