For the Linguine With Clams I don't think Nigella means - "Drain the clams, discarding those that remain open" (point 3)?
Nigella's Linguine With Clams (from NIGELLISSIMA) uses fresh clams to make a briny, winey sauce to coat the pasta. The uncooked clams are left to soak in a bowl of cold water while the first part of the recipe is prepared. At stage 3, when the uncooked clams are drained, Nigella says that any clams that are still open should be discarded. This is correct as the clams at this point are uncooked and still alive and any that remain open are likely to be dead, so eating them could lead to food poisoning. If you are unsure then rattle the bowl with the clams in, or tap the open ones against the side of the bowl. The motion will make them shut if they are still alive and if they do not shut then they should definitely be thrown away.
The closed raw clams should open up during cooking. After cooking you should discard any that remain closed as these could also be dead and not good to eat. This system should also be used for other similar bivalves, such as mussels in Mouclade.