What is this? On the signup page for your product, disable or remove all navigational elements that would enable a user to go back to the previous page. This includes disabling your site logo, which is often linked to the homepage.
Why should I do this? Preventing a user from backing out of a process is a good enough motivation for some users to complete the process. This can increase your conversion rate on the signup page.
Who else is doing this? Amazon is one of the most famous examples of having a checkout process in which you cannot freely choose to go back to the previous page - the progress arrows at the top are non-clickable and the only obvious options are to progress forward. Groupon and many group-buying sites also remove navigation elements on the signup page.