WIth #1, the placenta was anterior, he liked to hang around posterior all during the pregnancy. I don't know what position he was in when labor started, but I suspect he was still posterior.
I had nothing but back labor -- nothing.. Felt nothing across my belly, labor pains only were waves of cramps-to-stabs across my back, beginning about 1am when I started shedding mucus plug. I went to see the midwife to get checked out around 9am and was 4-5 cms dilated, so she suggested we hit the hospital. I hung out in the tub for awhile until I didn't feel like it anymore - preferred to rock in the rocker or pace. That turned out to be transition. My water broke at the end of that and suddenly I had overwhelming pushing urges -- uncontrollable, irresistable. Even with that, though, it took about 45 minutes of pushing to move him down and out, and I suspect that was becuase he had to turn to come out because he was posterior. No ultimate troubles though -- and the endorphins were so high that instead of a Ring of Fire I felt quasi-pleasant throbbing as he emerged.
#2 was NOT posterior, so was right in launch position. However, with her I figured out that some of my back labor is just the way I'm wired, because while I felt *something* across my belly, I still felt a lot in my back. I labored in the tub (hospital tub again), alone with just DH - and after only about an hour of that, just as I thought to myself "those last few contractions, they felt a bit different," and I found myself having to kneel upright during them to make them feel better -- I had a full-on fetal-ejection reflex -- one huge contraction, my water broke and she dropped to crowning all in one go, as I instinctively hopped to my feet.. Then I got all irrational, wouldn't sit back down in the tub, insisted on getting out, and she was born on the bed on the next contraction without any real pushing. She was 10#3 and I didn't even tear.
So: non-asynclic baby's birth was so much easier. Even my ill-positioned baby came out more easily than many babies, but my correctly-positioned baby? Pretty much fell out of me.