That rule is in the Cat Book (just after, "if just one piece of kibble is missing from the bowl, said bowl shall be considered empty").
Section III - Laps: (A) All laps are considered to be the property of the cat, and can be sat on any time they appear.
1. There shall be no time limit as to how long cat may remain on lap.
2. Once cat has occupied said lap, the person with lap shall not remove or scold the cat, not shall they get up (as to "remove" said lap) until such time as cat(s) have relinquished said lap beyond a period no less than three (3) minutes.
3. Telling the cat he's/she's handsome/beautiful, and treats shall be given to (lauded upon) cat in lap. Any pets, cuddles, or other touching are up to the cat. No human can tell the cat it is heavy and/or fat!
(B) Violators of this rule may/shall be issued a "warning" such as (but not limited to) - dispatching random hairballs (rug, shoe, bed, etc.), yowling at 3 am, knocking items off a shelf, and/or a hard resentful stare.
When a cat sits on you, whatever part of your body, and starts purring they are trying to heal you. My friend from Bulgaria told me that this was one of their old folk tails. Come to find out later researchers discovered that the same frequency of a cat’s purr can be used to heal bones. And it has been used for that since.
You don't HAVE to move. ALL you have to do is ANNOY your cat until HE WANTS to move!!! Blowing on his ears is good...usually works...or fidgeting and wiggling...and after a bit of tail lashing, the cat will jump down, and give you one of THESE looks!!!