Liquid soap is to make your solution stick to the leaves.I found this on another forum:
"Three large cloves of crushed garlic
1 tablespoon of vegetable oil
One teaspoon of liquid soap
One litre of water
Combine the garlic and vegetable oil and leave to soak overnight. Strain and add to the litre of water along with the liquid soap. Spray regularly. Garlic is known for its antifungal and antibacterial properties, but it is its insect repellent qualities that most gardeners like."
Im not sure about the liquid soap though, surely garlic and vegetable oil will be enough as a general insect repellent. Has anyone got any knowledge of using liquid soap as a detterent?