My wife and I tried it during labor and it was amazing. It didn't trigger a contraction, but it did make next contraction more intense, and last slightly longer. We experimented and this is what we found.
It was better to do both nipples at the same time, than to do only one, or one and then the other.
Getting the nipples hard and then wiggling them seamed to be the key. We found that a gentle flicking motion, or gentle pinch and rolling worked best.
It was more successful for me to do it to her than for her to stimulate them herself. Even though she did the same things, and nipples seamed the same level of erect, it was more effective if I did it. (Lucky me)
10 seconds of stimulation produced a noticeable increase in the next contraction's intensity.
20-30 seconds of stimulation was ideal for her situation.
60 seconds of stimulation increased the contraction way too much. Pain and discomfort was well beyond the reasonable level of normal labor contractions.
After 5 or 6 contraction cycles the effect faded. We would then go 5 or 6 cycles without the stimulation for her body to reset. Then the next 5 or 6 cycles with the stimulation were right back up at the intense level again.
It was a very useful technique to intensify the contractions, and enjoyable for both of us. Maybe enjoyed more by me than her, but overall a win-win-win. I highly recommend it to any mothers-to-be.