Block falling algorithm....

Hi all, been playing mr driller lately to study for a project, Im making a remake of it soon I was puzzled by how they make the blocks fall when the level is being re-adjusted, by that I mean when a block below ceases to exist, how do they make the others fall? I know how Id do it, but I dont get how they do it when u got connected blocks. To see what I mean check out the illustration below:

    1
    2345
      6
    X
 

say the numbers one to six are the coloured blocks (all with same colour and connected to each other) and X is another colour block. all the coloured blocks (the same number) should fall until block 2 sits on top of block X. like so

    1
    2345
    x 6

Mr Driller does this and I got no idea how I would design an algorithm to do this....blocks 3,4,5,6 and 1 dont fall all because they are connected to block 2 that is sitting on a block (x) which cannot fall. Thanks for your help Dark Star UK