how the pigment is placed
When the needles are immersed in the tattoo pigment, surface tension coats the needle grouping, holds an amount of pigment in the tube tip. When the needles are run in the clients skin the pigment is transferred onto the skin, and the needle grouping runs through it. When the needles penetrate the skin, pigment is carried into the skin on the microscopic texture of the needle shafts. When the needles are withdrawn from the skin, much of the pigment remains.
This is obviously where the pain associated with tattooing comes from. The needles cause a certain amount of trauma to the skin, but only to a very limited depth which amounts to a superficial aggravation to the upper layers.