To attach or secure with pins.
Pin down the tablecloth, so it doesn't blow away in the breeze.
To identify something (e.g. a decision or plan) clearly or specifically.
Something is wrong, but I can't pin it down.
To hold the opponent back-first so both his or her shoulders touch the mat.
To corner somebody in order to get a firm answer.
Let's try to pin him down on a price.