Mallet and heel cuttings are not commonly used but they do have purposes.
There isn’t much difference between the two and the mallet has less chance of errors or rotting so you shouldn’t use the heel version unless you have a specific purpose.
A mallet cutting will allow you to make a type of leaf cutting from plants that ordinarily won’t start from leaves.
Since the leaf cutting contains a portion of the stem with a growth bud it can be used for any type of begonia.
It’s mostly useful for creating as many plants as possible of a certain variety. Say you have a cane with one stem that has several nodes with good buds.
If you propagate by stem cuttings you might only get one or two cuttings. By using mallet cuttings you may get a dozen, depending on how many nodes and leaves there were.
Varieties of canes that drop their leaves easily may not be good candidates because the leaf may separate from the stem before the mallet roots.
Treat mallet cuttings as you would whole leaf cuttings following the same procedures. After rooting a shoot will grow from the bud on the cutting.
Credits / references:
Brad Thompson | Brad’s Begonia World