Propagation
This video presents the same text shown beside it, spoken and on screen. It adds nothing the text does not say.
State
Constraint propagation shrinks domains before and during search, deleting every value that no solution could possibly use — then repeating until nothing more can be deleted.
Show
One rule does the work: for each constraint, remove any value with no legal partner; repeat to a standstill.
Watch for
Propagation narrows the problem; it does not always finish it — the remainder still needs search.