When first coined, the expression “hitting on all six” was “hitting on all four.”
It may become, as we adjust our speech to the progress of the machine age, “hitting on all twelve” or sixteen or other multiple of two.
Whatever the alteration in the numeral, it means “functioning perfectly.” We get it, prosaically, from the shop mechanic who applied it to the cylinders of the automobile.
If the engine is running smoothly and efficiently, all the pistons in the cylinders, whatever their number, are hitting perfectly.
We have given it a figurative sense, too, and use it, for example, in compliment to a person who, we say, is “hitting on all six” when he is giving a forceful talk or an excellent performance.