The concentration begins with one-pointedness concentration and then when the concentration becomes strong you enter a sustained concentration which is a continuous concentration with no interruptions. You start to enter the remaining jhanic factors of aiming, happiness, and joy or rapture. You shift your attention from the meditation subject to the joy associated with your concentration. You do not cling to the sensations, but just watch them. The experience can include some very pleasant physical sensations such as goose bumps on the body and the hair standing up to more intense pleasures which grow in intensity and explode into a state of ecstasy. If you have pain in your legs, knees, or other part of the body during meditation, the pain will actually disappear while you are in the jhanas. The pleasant sensations can be so strong to eliminate your painful sensations.