One important effect of the premature depolarization is that the ectopic focus will depolarize the cells of the SA node, causing the SA node to skip a cycle.
But eventually, the resting membrane potential gets so high that it's above the threshold potential, meaning that once the muscle depolarizes and contracts, it can't repolarize to allow another contraction.
That process started by depolarizing the cell — that is, pushing the cell's membrane potential from negative toward positive, past a threshold that triggered voltage-gated ion channels to open.
Now, if we measure out the interval between each SA node fire, we see that it should have fired here, but it doesn't because it was depolarized by the ectopic focus.
Remember that at rest, cells are negatively charged relative to the slightly positive outside environment and when they depolarize, the cells become positively charged, leaving a slightly negative charge in the outside environment.