How do auxins promote the growth of a tendril around a support?

Explanation

The tendrils are sensitive to touch. When they come in contact with any support, the part of the tendril in contact with the object does not grow as rapidly as the part of the tendril away from the object.
The reason:
The auxin diffuses away from the part of the tendril that is in contact with the support towards the direction opposite to it. This concentration of auxin stimulates the cells to grow longer on the side of the shoot which is away from the support. This causes the tendril to circle around the object and thus cling to it.