If the Commons disagree to any Lords Amendments without suggesting an alternative they have to give a reason for their disagreement. A specially established committee decides what the reasons will be. It’s known as the Reasons Committee.
The Committee is established by a motion moved by the minister (or MP in charge of the bill for a Private Member’s Bill). This happens once all the Lords Amendments have been dealt with. If the bill has a timetable associated with it, the motion establishing the Reasons Committee is not debatable. If the Bill does not have a timetable, the motion can be amended and debated. The Committee meets immediately.
Business in the Chamber carries on as usual whilst the Reasons Committee meets. In practice, the meetings tend to be very quick and the reasons it agrees are normally brief. It reports its decisions to the Speaker in the Chamber, but no announcement is made. The reason for the Commons disagreement is recorded in the minutes for that day and sent to the House of Lords.