Why do you use the finger tips to whilst doing the rubbing in method?

You don't use your palm to do the rubbing in method.

You use your fingers only.

This is because you are generally rubbing in something like butter or margerine until it resembles breadcrumbs, and you don't want the butter/margerine to melt.

Your palm is the hottest part of your hand, so it will melt the butter, but your fingers won't melt it as fast, because they are colder.

This is why you only use your fingers to rub in and not your palm - if your palms are clean, you are cooking well.

Hope I helped. :)