Originally Posted by .alhamdulillah.
The reason for rubbing the limb whilst water is flowing over the limb is to be assured that the water has made contact with every part of the limb.
Rubbing also makes sure that any subsidiary particles of dirt do not hinder that water touching the entire limb.
So, taking you question into account, rubbing between the toes - serves both of the above purposes. Also by rubbing the finger between the toes, takes out the dirt. You will notice there is a difference of opinion as to when one should rub the between toes (Start, middle or end of the procedure of washing the feet). In reality it will not make a big difference as long as the entire foot is wet as a result.
I'm going to give you a little tip whilst on the topic. The best way to understand the Masaa'ils of wudu is to use a jug and not a flowing tap. When using a flowing tap, both hands are free and water is always on flow. This might be much easier i agree, but i guarantee you most people who use a flowing tap won't actually know all the masaa'il.
Example: Most people who make wudu with flowing taps (probably all their lives) fall into a mistaken understanding that one needs both hands to wash the face. When you start using a jug/vessel you will come across complications as both hands are no longer free, it will then raise further questions.
Take your time in wudu. The ikhlaas of your wudu is the ikhlaas of your Salaah. Using a jug also saves water and because wudu becomes a slower process with a jug - it makes you think about what you are doing. Using taps, the wudu happens so fast, that we cant even recall making that wudu hours later.
Just to show you how much water is saved, I carried out a little experiment.
I opened my tap and made what i regard as a standard speed wudu and i timed myself on a stopwatch. Lets say it was 3 minutes.
After that, i let the tap run without adjusting the flow for another 3 minutes and put a bucket with measurements in litres under the tap. Hence the flow rate remained the same.
I recorded it at between 6-7 Litres. I use a jug for wudu, and i only use 1.5 Litres. That means roughly a tap wudu is 3 times more than a jug.