1. By skin application (i.e. massage, ointments, creams, lotions, compresses, etc.):
During the course of treatment, a certain amount of the essential oils is absorbed through the skin into the bloodstream. The power of penetration is great; the time of absorption may be between 20 to 70 minutes.
Each essential oil has unique properties that can be used to stimulate physical processes such as elimination, circulation, and digestion or to ease muscular tension, aches, pains, etc.

2. By inhalation (i.e. bath, steam inhalation, valorization, personal perfumes, shower gel, room spray, etc.):
Essential oils are highly volatile and so they can also enter the bloodstream through the lungs when breathed in.

The fragrance of essential oils, like any other smell, affects our olfactory sense (sense of smell) - the stimulus is received by the region of the brain associated with emotional response and mood change. Essential oils have been observed to trigger favourable emotional response, which usually contrast with the way the individual felt previously.
This might be the first step towards making positive change when one is highly stressed!