Tension is nothing more than throwing otherwise good balance awry.
I must admit I don't find examples of how to design advertisements for "Bradley's Lawn Care" all that exciting, but I can see there are some good principles being taught here.
However, I could find no mention of tension resulting from exclusion. Seems to me, one can exclude something in a picture by two methods; cropping it out or covering it up. The former applies to Michael's shot of the dunes, the latter to the girl portrait. I wonder if Howard would feel the same way about that shot if the eye obscured by hair was instead a b r e a s t
. (Sorry if I seem crude. I'm really trying to maintain my usual high(?) intellectual standards.)
Of course, the striptease is the perfect example of tension being created by covering up something. The very successful fashion item, the bikini, also relies upon that technique.