I agree with McGrath and Dave Bryant - I don't stick a finger in my ear but I do press on the outside to block out external noise and hear what I'm singing - and, since we used to have 4 of the people Dave mentioned as members of Herga, it is a jolly useful technique for checking whether what you're singing fits with what you're hearing in your other ear. Only works if you're sober, though. (I thought I should point that out before anyone else does on my behalf.)

Kitty