It is a long skinny blanket with a pocket for legs and fabric wings to wrap around the arms (we used to lovingly call it the 'baby straight jacket' for my DGDs

) Basically you put them in the pocket, put the 'wings' around their arms which hold them still and then wrap them up like a burrito.

When my oldest DGD was born, they started putting it on her immediately. She loved it. By the time she was about 6 months and would get fussy, all she had to do was see them pick up the blanket and come towards her and she would instantly calm down. She absolutely loved it. She was very collicky (sp?) and I don't know how they would have made it without it. Here is the link to the original miracle blanket. There is a video that shows you how to wrap them.
http://www.miracleblanket.com/
When she got pregnant with the 2nd, I used the original as a pattern and made more. She loved it too but not for as long. I have made several since (because you can make them out of all the cutsie flannels) and give them for gifts.