막다 is stopping something getting through, and it shouts well: 막아 막아! is what you yell at a defender, three times over in these lines. Two compounds appear: 틀어막다, stuffing or clamping something shut — 입을 틀어 막은 거예요, a hand clapped over the mouth in shock — and 돌려막기, literally blocking in rotation, the word for covering one debt with another or recycling the same material over and over.
Register: casual; the shouted use is from games and sport