Complex sentence: The contestant and chaser are presented with the same multiple choice question, and each locks in their answer, which cannot then be changed; the other has five seconds to answer after them, otherwise they are locked out and do not advance on the gameboard.
Split into simpler sentences: The contestant and chaser are presented with the same multiple choice question, and each locks in their answer separately. After one has selected their answer, they may not change it and the other has five seconds to answer after them, otherwise they are locked out and do not advance on the gameboard.