Willow water rooting hormone works for a very basic reason. The bark of all Salix species contains salicylic acid, which has been used for centuries as a home remedy for pain relief, but it also works as a rooting enhancer for plants.
If you have a willow tree of any kind in your garden, give it a try to root some kind of plant or other. If you don't have another option, this is one that will work for most plants.
Here's how to do it:
Cut some smallish twigs, about the size of a pencil, and cut them into four inch lengths. Put them in a jar, with a spoon in it so it doesn't shatter, and pour boiling or near boiling water into it to cover the twigs completely.
Put the lid on and put it in a cool corner to steep for a week or two. Then put your cuttings in, leave them overnight, then stick them into a pot with soil.
If you want, keep the jar on the windowsill and just add the cuttings to it and watch the roots form. The use of water propagation taken to the next level, with yet another way to utilize jars with lids that aren't canning jars.
In another twist, you can simply dissolve an aspirin (not an enteric coated one though) and use it the same way. The chemical that's in willow bark is almost identical.
This might not be the very best rooting hormone, but in a pinch it will certainly do the trick.