A motion is put "That the House do now resolve itself into a committee upon the bill". This can be opposed, but its agreement is often a mere formality.
When this is agreed to, the Lord Speaker (or whichever Lord is in her place) leaves the Woolsack and sits in a modern chair at the Table of the House. Rules are less formal for the Committee stage. Frequently the committee stage will be interrupted, as it was yesterday on the fifth day of the consideration of the Climate Change Bill, for 'dinner break business'. The Woolsack was reoccupied for this.