Magnesium citrate is a saline irritant laxative that also inhibits the ab- sorption of water from the intestine.
Magnesium Sulphate is a Hyperosmotic saline type laxative. This means that the magnesia acts to pull a salt or saline solution into the intestines from the blood. This excessive fluid then provides a flushing or laxative action within the intestines.
Magnesium malate is a highly biologically absorbable form of magnesium salt. In high doses the magnesium will provide a laxative effect. I don't know what the dose rate is and I do not know if it purges the stomach. It needs to taken with liquid and I think it could work out quite expensive. The malate although good to consume at any time is readily available in good quality fruit vegetables; even more natural.
At the time of flushing the action of malate is too late to "loosen up" the stones