Stake size and margin required

I have been trawling through posts on the forum and in one there was a link to tradertom.com

He offers live trading alerts - when you drill down to his swing trading he has said he grew £100k to £1.5k over 6 months. No reason to doubt him at all what I don’t understand is where he quotes the stake size and margin required.

So for example on the Dow he has put stake size £100 margin required £15000. How does this work???