I also did it at 8 months to get my little one to go to bed at night. I never let her just cry it out in the middle of the night without checking to make sure she wasnt wet hungry cold ect. I tried the cry for 1 minute then went into her room and rubbed her back but didnt pick her up and increased length of minutes as the night went on. It didnt work for us. I had to just let her cry it out without going in her room. First night was 1 1/2 hours and was horrible for me! I balled the entire time. Everynight got better and better until she finally just went right to sleep everynight without crying. That being said i have a video monitor and i wouldnt have been able to do it without being able to see she was safe. Good luck!
If your driving in heavy traffic and can't attend to them until you have a way of safely stopping somewhere. If you were just holding them and sat them down knowing they are perfectly fine, just upset that you put them down.
I did with my son at about 8 mmonths. He sleep so much better after that! He slept through the night and took better naps too. It was pretty hard to just listen to him but we only had to do it once.