They make a signature design and ask you to put into your signature area. You have to post a number of post which are constructive in order to recieve payouts. When people browse through threads, they are likely to spend more time looking at the more constructive post and usually look at the signature too. This is a good method to introduce useful post to the community while discouraging post spammers. Compared to forum advertising which cost a few BTC and only displays your ad in rotation for one week, you can use 30BTC for 200++ people to put their signature on their signature space for one month and post constructively which is more cost effective. Since this ad change applies to every single post, those browsing older threads would see the signature also.