Cream of tartar is acid. It is an ingredient of baking powder along with bicarbonate of soda, which is alkaline. when combined with liquid and heat the result is 'bubbles' - making the crumpets, cake, muffins, scones etc. rise, even without any yeast.
If your crumpets don't have holes in the top, you haven't cooked them long enough before lifting them, like making pancakes.