Yeah I set it really low so that the change is almost invisible with each individual stroke, but as you layer on more and more it slowly builds up. I also use the brush in an almost stippling fashion to make sure it creates a gradient.
I have it set only to the brushes diameter, but that's also because of how I use the brush. If I had a more flowing hand i probably could use the pressure sensitivity with the opacity and flow as well, I just don't like to.
The erasing part sounds like the masking layer. Did they switch to the eraser or did they stay with the brush. Also did the color bar turn into a k with only black and white. Then the person is using a masking layer.