Hey Gordenia. It is a proven method of creating a tree that produces more than one varieties of the same fruit on a hardy rootstock! You find a tree that grows well,is resistant to drought or too much rain ,diseases etc and you graft the varieties of the same fruit or same family of that fruit( peach, plumb,apricot, almont,nectarine belong to the same group and are compatible to graft together)that you like or match your requirements on it! That is the improvement! Hope I could help!
Here are a couple of successful and one that did not seem to make it!