I have some experience with boarding. I don't board my dog as he is allowed to come with me, but my trainer boards a lot of dogs. Most of the people come and take their dogs away and don't notice a difference because they didn't do the training. The dog also doesn't always understand that what they were "taught" applies to YOU as well. To them - you are back and they are happy to go back to their old ways.
A lot of people that send their dogs away for "professional" training usually don't have any idea how to handle their dogs when they return. It depends a lot on the duration your dog is away, what they are being taught, and how they are being taught. It's something that you will need to continue at home if it is going to stick.