The previous question is a motion asking the House to reverse its decision to debate whatever is currently being debated.
To move the previous question you need to say, ‘I beg to move that the question be not now put”. You can move the previous question at any time during a debate, but you must already be speaking at the time—you can’t interrupt another MP to move the previous question.
The motion on the previous question can be debated (and that debate can itself be subject to a closure motion). If the motion on the previous question is agreed, the debate that it interrupted ends without a decision and the House moves on to the next item on the Order Paper.
If the motion isn’t agreed, the original question that was being considered when the debate was interrupted must immediately be put to a decision.