After centering the clay a depression is made in the centre which is then opened out using the fingers in a grip, leaving a suitable thickness for the base of the pot.
Lifting the walls of the pot can take many attempts to master. The finger tips inside are held just above the finger or knuckle on the outside forming a ridge which is then lifted upwards, both hands synchronised and working together
If the fingers pinch the clay walls too much the pot may tear.
During the process of lifting it is important to keep the upward movement angled slightly inward to counter the tendency for the walls of the pot to flare outwards.
Some potters prefer to use the side of their finger or knuckle to do the lifting on the outside. I’ve seen others use their finger tips. But once you understand the method and how exactly the walls are being lifted up and thinned, by grabbing the clay from the bottom and creating a ridge which can be coaxed upwards to grow the pot, the choice of which part of the hand touches the pot is a personal one.