In our society this rule requires that one person try to repay what another person has provided. This might be a little overused on web sites these days, but applied with finesse it works well. Especially if the free gift has at least a perceived value. The obligation to repay lingers, and kicks in when you ask for the commitment. This is evident in sites offering a high quantity of very hard to find and valuable information.
Visitors use the site daily, and the pressure to repay is mounting.
Then comes the request for a commitment. No one even thinks twice about using it as justification to comply. Establishing credibility on the web is key to success. If a site visitor is uncertain about your credibility, pointing out that several of his or her colleagues, people just like them are happy customers will trigger this shortcut to compliance. That's why testimonials are so important to include on any web site.