Follow the invasion squares

 In order to manipulate the killzone, you need to learn to recognize the invasion squares.

White to move

r4nk1/pp3rp1/2b4p/3p1p1Q/5q1B/3B2R1/PPP4P/4R2K w - - 0 29

The first invasion square is e7. Why? Because B and R converge there.

The second invasion square is g7. Why? Because Rg3 and Re7 converge there.

What does the queen sac Qxf7+ achieve? It eliminates the defender of both invasion squares. What is more, it does so with tempo.

The task at hand is to recognize the potential invasion squares (count the attackers relative to the defenders), and to find the tempo moves that change the balance.