The nutshells contain saponin - a natural detergent that has cleaning and degreasing properties - which is released when the shells come into contact with water. For this reason, soap nuts are a non-polluting, compostable alternative to conventional laundry detergents and synthetic soaps: free of foaming agents, bleach, phthalates, phosphates and parabens, they are 100% safe and don't cause any allergic reaction.

People can, in some circumstances, find soap nuts a little disappointing. In my experience, they are less effective when the laundry is very soiled (items we have to wash at 50°-60°C) as they don't cope with stains very well. However, soap nuts are particularly effective when washing fleece, silk and wool, which become really soft and nice. They are perfect for a coloured load or for delicate fabrics that require only a gentle wash. I suggest washing these fabrics at 30°C or in COLD water, a great habit of saving energy too. 

Some people don't like soap nuts because they don't make their clothes smell "nice", as washing powder does. Personally, I don't like chemically-perfumed garments, because the "fake good smell" of the fabric conditioner reminds me of how bad it is for us and the environment. I will admit that soap nuts do have a strong smell, but it will disappear from your clothes as soon as they are dry.

I have found that the best way to use soap nuts is to boil them and produce a liquid soap, instead of putting a few nuts in a wash-bag as many suggest.