By definition, a pencil turn is a turn on the spot with feet held together. There is nothing in the definition that implies where the weight is held (eg one foot or both feet, over the ball or over the heel), but in real life most people turn with the weight held over the ball of one foot.
If you think about it, it would be difficult to keep the feet together if the weight is truly distributed between them. When both feet are fixed to the floor, they turn independently of each other, and so throughout the rotation their position relative to one another changes -- feet that begin in 2nd position, for example, end in 4th position after a simple quarter turn. In order for feet to remain in 2nd position, one foot has to be free of enough weight that it can move along the floor, and therefore keep its position relative to the primarily-weighted foot. The same goes for feet in first position.
Take a look at the spins & turns lesson on this website for some hints on how to improve your own turns.
Regards,
Jonathan