A double strength infusion is when you use twice as much herb as the single infusion. You usually use this when you have roots, barks, and seeds. It takes about 15-20 minutes of simmering to get the benefit from these woody parts. This term generaly refers to a herbal solution in alcohol. The advantage of tinctures is that is has a much longer shelf life.
Alcohol is also able to pull the healing elements from the herbal plant parts.
Many herbs have relaxing and healing benefits and are used in this way. These oils are extracted by distilling the plant. These essential oils are very strong and can be toxic. Care must be taken when using essential oils. This process pulls the medicinal properties out of the herb and puts it into the solution.