Colchicine is a mitotic inhibitor--arrests dividing cells in metaphase if I recall correctly. It is primarily used to treat gout, as it inhibits the movement and function of the white blood cells that cause all that pain & inflammation. In plants, it's supposed to arrest the cells in metaphase, when the (doubled) chromosomes are lined up. In the cases where conversion takes place, the colchicine is removed and the plant goes on to double all those chromosomes, which are already doubled, and now you end up with 2 complete sets before the cell divides.
The level required to induce polyploidy is very close to the lethal level (for the protocorm), which is why conversion rates aren't very good. In fact, some people like to advertise "colchicine-treated" for a nice cross when they use very little colchicine to get the maximum number of seedlings, and of course none of them has any chance of being polyploid.