Age group(s) for which this technique is helpful:
Preschool and Younger
Elementary School
Middle School

Primarily used to improve behavior:
Both At Home and At School

Instructions for using this technique:
What I have found, through the advice of others, is to use a timer when the child is doing something they enjoy. This seems to make the transition a little easier and they don't spend too long on something (like Nintendo for example). It's a good idea to have the next project ready to go (or at least tell them the idea of it) when the first project ends. Let them know when they have 5 minutes left, then 2, and so on. I've found rather quickly how if done consistently, it will diminish the "meltdowns".