Don’t just keep the finger straight as you go. Not only do you miss major pleasure centers, but you might actually hit the cervix, which may result in pain or discomfort. Curving the finger upwards means it achieves optimal friction over the G-spot and, is you reach deep enough, slides easily into the anterior fornix to stimulate the A-spot. If you don’t know, that’s the pocket just above the cervix that the penis would optimally slide into when doing front-every PIV (missionary, cowgirl, etc).

This is assuming the person is on their back. The curve should be towards the anterior (front) side of their body.