As a knife dulls, it may not do the job at hand and require more pressure with every cut. As you increase pressure, it may be a little harder to control the blade and one slip is all it takes to discover that dulling blade is still sharp enough to cut your skin!
Always on the quest for the ultimate in sharpness and failing miserably, I've scoured many websites and videos for THE information. One constant I always see is instructions to keep the blade at a 15-20 degree angle to the sharpening stone. Another way to look at that is the angle is about the same as the angle of a book of matches.
At first glance, the accompanying video may be off-putting to anyone outdoors-minded but, hang in there because I've found these instructions to be invaluable!