inasisi - 7:02 pm on Feb 2, 2005 (gmt 0)

Impression spamming is when a competitor pauses their campaign and then runs a program that searches Google for the particular keyword thousands of times. This program does not click on any of the Sponsored Results as that would be easy to find through Fraud detectors. By doing this your ads has got thousands of impressions but no clicks. Thus your keyword could be disabled or would have a very low CTR. Thus when the competitor resumes his campaign, he has a much better CTR than you and consequently a better position.

I have not experienced this first hand but it is definitely possible. I am not sure if Google has good fraud filters for this kind of impression spamming.