Overloading

Nb6

White's knight on c3 is the sole defender of two pieces: the pawn on a4, newly attacked by the knight on b6, and the bishop on e2, attacked by the black queen on f2. If either is captured, the knight cannot recapture without abandoning the other. Before Nb6 the knight guarded only e2, so the a4 duty is new.

Nb6

The nearest neighbour is the same defender guarding only one attacked piece. That is ordinary defence, not overloading. The motif needs a second, independently attacked piece behind the same sole defender. If either the second attacker or the second attacked piece is removed, the overload disappears. No sibling entry covers the remaining position, which is the negation of the motif.

With a second defender on a4, the knight on c3 is no longer its sole guardian, so that duty no longer counts toward the overload. The knight on c3 is left with only one duty, e2, and a single duty is not an overload. This position is the negation of the motif. Sharing one of the two duties ends the whole state, and no sibling entry covers what remains.