But we have to be careful about the way the three cells are aligned. All on one row or column will not do.
More precisely an XYZ-Wing has three cells that contain only 3 different numbers between them, but which fall outside the confines of one row/column/box, with one of the cells (the 'apex' or 'hinge') being able to see the other two; those other two having only one number in common; and the apex having all three numbers as candidates.”
It follows therefore that one or other of the three cells must contain the common number; and hence any extraneous cell (there can only be two of them!) that "sees" all three cells of the XYZ-Wing cannot have that number as its true value.