The four-three (4-3) is the strongest shape a single stone can make: one point that completes a four AND an open three at the same time. Why is it unstoppable? If white blocks the four, the open three becomes an open four; if white guards the three, the four simply becomes five. One stone, two threats, one defender. Find it yourself here: black's diagonal three is capped at one end, and a young pair waits on the column beside it — one point turns this into a four and an open three together.
Finish this step to move on.