I was having trouble with him immediately spitting the baby carrots out without even letting them hit his tongue. He'd hold them in his teeth, curl up his lip and generally be a snot about it. Holding his mouth shut probably wasn't the ideal approach, but it did work and I wasn't being really aggressive about it. If he wanted to he could have pulled his head back over the fence. Once the light bulb clicked though, he would chew first and ask questions later. I had to stop giving him treats because he became such a butt about it.
Again, I would never do this to a baby or an unhandled/headshy horse. Or any other horse, really. Or again, if I had to do it over. :)