When you specify a KEY JOIN and put a join condition in an ON clause, the result is the conjunction of the two join conditions. For example:

SELECT *
FROM A KEY JOIN B
ON A.x = B.y;

If the join condition generated by the key join of A and B is A.w = B.z, then this query is equivalent to

SELECT *
FROM A JOIN B
ON A.x = B.y AND A.w = B.z;