The classic smothered mate involves a king against the edge of the board surrounded by its own pieces. But the same pattern can arise when the empty squares around the king are under attack and thus off-limits as flight squares. Here is a simple study in the principle. If Black starts with the implausible-looking Qg1+, White has to reply RxQ (the king can’t capture because the queen has protection from the rook on g6). But see that now White’s king is invisibly smothered: its only flight square, g2, is made off-limits by that same Black rook. So now Black mates with Nf2.