"Common keyword technique" refers to the grouping of at least 2 common keywords into an adgroup. It will give you a higher CTR (Click through Rate) and provide you with a higher overall Quality Score. For example: Campaign Name: ckt Ad Group Name: Make Money Keywords: make money at home [make money at home] "make money at home" make money from home [make money from home] "make money from home" make money internet [make money internet] "make money internet" make money online [make money online] "make money online" Your Ad could look like this: Make Money From Home Want To Make Extra Money? Learn How I make $1000 / Day ckt.com As you can see, all keywords within the ad group have 2 common words "Make money". Also, you can see that the Ad has the common keywords in the title, and body of the ad. This will mean that when a search is done for How to Make Money Online, this ad may displayed. Also, please note that the name of the ad group is based on the 2 common keywords as well.
Why should anyone care about the ad group name? As far as I know, the ad group name has no effect on the quality score.
It also helps when transitioning accounts. I realized that the first time I was handed an account with about 2K ad groups and quickly realized that the ad groups names were essentially random and had no relation to keywords they included. "AG1774" right next to "070707T". Figuring out what was what was a pain.