the 1 in the middle right box must be in the yellow square.

Here is a different example.

The 6 in the top row must be in the yellow square.

**Terminology** A forced digit is often referred to
as a 'hidden single'. (This 'naked' vs. 'hidden' stuff refers to the situation
where one has written down all possibilities for all fields. Now if there is
only one possibility somewhere, that is immediately apparent, while it
requires a scan over a row or column or box to recognize that some digit
can only be in one place. But in practice it is too time-consuming to
write down all possibilities, and forced digits - hidden singles -
are found more easily than forced positions - naked singles.)