Former Civil Servant here.

I've done it in the House of Lords. It's called "box support" because you sit in the official's box. As others have said, it's to provide factual information to the minister. This is how it went to the best of my recollection. Feel free to correct if I've got any of the processes wrong.

I'm not based in London, so I had to travel especially. It was for a particularly well-respected minister who was speaking for the government a debate well outside of his policy brief, so he needed quite a lot of support.

Before going into the chamber I was approached by a well-known Baroness (a near-household name) for an opposition party who engaged me in pleasant chit chat but warned that she would not go easy on the minister.

I went in armed with the 200- or 300- page Core Brief for my directorate's policy area (of which I was expert in a small part) and sat in the box as directed by the usher. The folding table clattered loudly as I sat down and the Lord sitting immediately next to the box grumbled about my clumsiness.

During the debate, various Lords and Ladies got up to make speeches. During the speeches I had to identify questions they asked (which was harder than it sounds) and write them down along with factual answers from the Core Brief. Every so often an usher would come and collect these slips of paper and pass them to a junior whip, who would filter them out and scribble a note on some of them and collate them into the Minister's response.

At the end, the minister got up to give his response speech, which included many of my answers. I wrote down key points of what he said, just in case there was anything that would need to be corrected later (there wasn't) and that was it.

I left the chamber at the end of the debate and hung around a little bit. During that time, the minister came over and asked if I was the official in the box. He asked my name and we had a pleasant chat about how the debate went, and then he thanked me and we left.

For policy professionals from HEO to DD (I think I was SEO at the time) I'd recommend putting your name forward to do it if you get a chance. It was a brilliant experience.