The method is to just move your attention from the location where the pīti feels the strongest to an area where it does not seem to be occurring, such as down an arm. You are not trying to move the energy; you are only moving your attention. Bring your attention back to the place where the pīti felt strong, and then notice whether the energy spread to the area where you moved your attention. Continue doing this in a gentle, unhurried way until the pīti and sukha fill the whole body.