Have you ever read a book where the character made a plan, but the author did not tell you what the plan was? This is suspense in plans. (Notice I didn't say 'planned suspense', because all suspense in a book is planned. :) )
It's very helpful if I want to build suspense in a story, because it keeps the reader wondering what they're going to do, and because it is very simple to do.
When I do this, I give a couple hints as to what my characters will do. A thought, a small event- it makes the reader more eager to find out what will happen.
The reader reads on, and the plan unfolds before his or her eyes.
Suspense in plans keeps the reader interested, and makes he or she want to keep reading, and that's one of the best things suspense in plans does.