I have had some success with PowerPoint animations. Put up the equation, and for each mouse click surround the symbol(s) you want to explain in a red box or som

I have had some success with PowerPoint animations. Put up the equation, and for each mouse click surround the symbol(s) you want to explain in a red box or something and have a infobox show up (which can also contain images, such as plots - very helpful). As you transition to the next symbol(s), fade away the previous infobox and surrounding box.