pick off play

Question
Runner leading off first, pitcher throws over.  Runner trying to get back ends up with his foot on the first baseman's foot, which is on the bag,  as the tag is made.  Is he safe or out and what rule covers this?

Answer
Gary,

Technically, the runner would be out, as he is not touching the base. Thinking of it another way: if a fielder places his leg in front of the base a runner is sliding into, the runner would not be touching the base.

However, it's common to still call these as they are (in effect, ignoring the fact the runner is on the fielder's foot). The simple reason why is that there is no way for an umpire to be 100% certain no part of the runner's foot is touching the base. Therefore, the assumption is that SOME part of his foot was on the base.