The Early Check
This video presents the same text shown beside it, spoken and on screen. It adds nothing the text does not say.
State
A constraint is tested the moment all its variables have values, so a doomed branch dies at the shallowest point the constraints allow.
Show
Assign a teacher two rooms in the first two choices and the violation fires immediately — no need to finish the timetable.
Watch for
Early checking prunes branches, not possibilities — the doomed subtree must still be entered to be caught.