algebra precalculus - As a child i used to do multiplication through this method. Now i want to understand the logic behind its working. Help! please

As a child i used to do multiplication through this method. Now i want to understand the logic behind its working. Help! please As you can see from the image, i lined numbers and subtract them from their nearest 10th number and multiply the difference and add the original numbers put them together and cancel the first digit. it worked always but how?

!http://i.imgur.com/ct1kami.jpg?1