On the about page it reads:
We use a health vocabulary database of hundreds of thousands of health-related concepts and relationships to suggest alternate search terms and concepts. This allows you to see the relationships among concepts and search terms that are similar, more general, and more specific than those you submitted.
And the first thing that pops into my head is MESH! So I thought I would give a whirl. I typed in a topic that we get a lot of questions about for papers -- which is overweight and limited it to Kids and Teens.
My big disappointment with the results was where the search engine derived it's definition...it used Wikipedia for it's authority. Great Ghost of Hippocrates! Why on earth would they chose to use Wikipedia's definition as opposed to the MESH or even Webster's Dictionary (which is what NIH/NLM uses). Which one has a great chance of changing faster or erratically? Do you want risk when you are search for health information?
They do provide a small link to related terms at the top of the page Obesity and Body Weight -- which is very important. The results themselves seems to be a mix of government, hospital, and "health sites".
Over all for a product still in the beta phase this search engine has a lot of potential -- although it is duplicating a lot of work that NIH/NLM does. But there's always advertising money to be made. It'll be interesting to see where this site goes.