Voicethread can be a powerful tool for advancing student's understanding of math concepts.

A growing set of applications of this tool is demonstrated by the K-7 Mathcasts 500 Project and http://www.mathplayground.com/mathcasts500.html

Young mathematicians can explore and communicate their thinking about math. They can do this in writing, speaking and with drawings.
They can do so with an authentic audience.
Their comments in the threads can serve as authentic assessments-- assessments that can be used to guide next steps and prompt self-reflections.
More on Voicethread

http://voicethread.com/library/29/ for discussion of above!

or, the direct instruction approach...

International Math Cooperative Voicethread