http://www.theforgottentechnology.com/Page6.htm

... ie you place two rocks/whatever underneath the slab near to the balance point, allowing you to then make use of a top-mounted counterweight to bring the slab up onto only one of them.

At that point, you can rotate the slab in a horizontal circle, and (being offset from the midpoint) the slab makes some progress in one direction of another. Like walking a fridge.

There are some MPGs, which aren't linked in but appear in the source of the page:

1
2
3
4 (note position of bucket on the ground at left hand end, relative to end of slab, at start and finish).

Having moved a 9 ton gate made of rock, at the Coral Castle in Florida, with just my little finger I can testify that - if you get something heavy balanced just right, you can move it very easily.

The trick is not to try and lift dead weight - make it lift itself!
--
simon