Never stuck my finger in my ear while singing (and try valiantly not to do ao at other times!) but I DO occasionally find myself with my index finger pressed semi-firmly against my jawbone somewhere in that region, when trying to sing harmonies in noisy environments! I could speculate about picking up harmonics (sic) or feeding them back around my head - but speculation is what it is. I certainly don't think its really a matter of blocking out the background - rather of amplifying somehing closer!

Regards

p.s. I think I should go and lie down....