Main Drain

I have a question about the operation of the main drain. I have a three old pool, and until recently, have never tried to operate the main drain. From what I understand, in order to do this you have to close that teardrop shaped flap on the bottom of the float valve in the skimmer. I have tried this, but I do not notice any suction coming from the main drain. Would I notice the suction, like you would with the vacuum, or would it be so subtle you would not really be able to tell? The pump does not seem to be straining when I try this, but it does not appear that the main drain is sucking in any water. Are there any tests I can do to see if I have a clog in the main drain? Does any one have any other suggestions on how to get the main drain operating correctly?

If you plug the skimmer and water is still being drawn into the pump (watch through the strainer basket window) and most importantly - continues to come out the return jets- then the main drain is drawing water. The other test would be to squirt some food coloring out of a bottle right above the main drain... does it get sucked into the drain?