Pin down

Pin down the tablecloth, so it doesn't blow away in the breeze.

There are approximately 1,000 places on British Railways where wagon brakes must be pinned down before trains proceed over severe falling gradients; the trains have to be brought to a stand for the brakes to be pinned down, and subsequently have again to be stopped for the brakes to be released.

Something is wrong, but I can't pin it down.

Let's try to pin him down on a price.