Rooting compound is used when propagating plants from cuttings. It stops the cut you make from healing and also prevents fungi, bacteria and viruses from affecting the plant.
The rooting hormone that is used in these compounds is normally from the willow plant. It's called IBA (indolebutyric acid) which belongs to the Auxin family and it helps stimulate root growth in plants, which is particularly important when you are propagating plants from cuttings.
Off-the-shelf rooting hormones can come in powder or gel form. Some contain not only the hormone IBA, but also vitamins and nutrients to assist with early root growth, and sometimes also fungicides to protect the cutting.
Dip the cut end of your cutting into the compound gel or mixture and then insert into the rooting medium (the medium into which you're planting the cutting). If using a rooting hormone powder make sure that the stem you dip into the powder is wet. This will make the powder cling to it better. Make sure that the hole in the rooting medium is big enough that it doesn't rub the powder off the stem.
Willow contains the active ingredient indolebutyric acid or IBA, which we can extract by placing willow twigs in boiling water.
When you're ready to use your solution for rooting plants, decant some into a separate container so you don't contaminate the main supply. Let your cutting stand in the solution overnight to absorb it, and then plant it in your rooting medium of choice.
I haven't been able to find solid evidence of this being effective. The idea is that because Aspirin also contains IBA and is made from willow, it should work well for rooting. I have read that you can dissolve an aspirin tablet in a cup of water and use this mixture as a rooting hormone instead of using willow bark.