Subject: How do two waves destructively interfere, only to reform and move on?

My physics teacher showed us a movie in which two transverse waves 
destructively interfered with each other.  For a moment, it apppears that 
the medium (a string) is perfectly still and that there are no more waves.  
However, the waves then reform and continue on their way.  My question is:  
What's going on!?  To clarify, I can't understand the energy transfers going 
on while this happens.  I've searched your website and can find no answer.  
If you could please answer my question.  My teacher said that he didn't 
understand because it is like energy canceled each other out and then 
reformed, but I don't really think there is such a thing as negative or 
positive energy.  Please help!  A detailed answer would be very appreciated.
Thanks,
Zoki

Re: How do two waves destructively interfere, only to reform and move on?