moptop has it spot on for me......."the bit just around the base of the glans, where the foreskin was removed, can be really really sensitive... " my g/f made a wonderful discovery recently when she got a little carried away during the rapid build up to a massive , and long delayed orgasm (for her) , she was fondling/squeezing my balls/cock as i tortured her clit and nipples relentlessly untill she was unable to hold back any longer...... she seemd to lose control , and as she started to take on that shudder of the pelvis , the one that tells me she's almost there, and her eyes unfocussed and the "bad" words spilled out through clenched teeth..... she began clawing at my cock with her long, and pretty sharp nails, catching me right under the lip of my knob, many times as she drew her nails up my cock from the base, and over my knob repeatedly, as there was plenty of oil/lube knocking around the sharp nails slid easily over the tuaght skin of my engorged cock, the sensation it brought cought me quite by surprise and brought about a huge tidal wave of an orgasm ( for us both), i would say it was in my top 3 EVER.
Moptop, you are spot on , at least for me

how she would achieve this in fellatio i don't know, but i can hope.