Sometimes it's about the empty places
My second blog on the nature of, let's call them level-2 tactics; those which are a bit harder to find than simple pins, skewers, etc.
Some more advanced tactics are often based around an empty square rather than straightforward attacks on pieces. A good example is this problem. In this position there are no immediate captures which win but looking more closely shows that black attacks the a2 square with 3 pieces while white only defends with two - this means black can think about putting a piece on this square. Once this idea is combined with black's attack on the f2 pawn it becomes obvious that white cannot allow black to sieze the 2nd rank even if he must lose the queen to prevent this.
I'd like to briefly share a similar idea from one of my own recent games, which shows a pin on an empty square. In this position I can play Rxb3+ because the a-pawn is pinned to the empty a1 square - capturing the rook would allow Qa1#. Note, white should have played the king to a1 which would only lose the bishop - the blocking move actually played loses quickly.
Sometimes looking at the empty squares is as important as looking at the pieces when looking for tactics.