From what I can gather it's a Buddhist in, and it's name means something like "the in of becoming all things." The text describes how to do it, but I don't totally get the description. From another source I gather that the right hand goes on top of the left hand and the palms face each other, then there's more description about what to do with your fingers. I don't know what the English word for in would be, but it's what they use in Naruto to refer to the hand gestures/signs/whatever, if that helps.