hq9+

What is the 'accumulator' in HQ9+?

I was just reading a bit about the HQ9+ programming language ( http://esolangs.org/wiki/HQ9 , http://en.wikipedia.org/wiki/HQ9%2B and http://www.cliff.biffle.org/esoterica/hq9plus.html ), and it tells me something about a so called 'accumulator' which can be incremented, but not be accessed. Also, using the + doesn't manipulate the resu...