Illustrator: Why pencil tool is not modifying/changing the selected shape

I have a shape selected. I want to change the path of that shape so without losing the selection of my shape, i press N for pencil tool, I start inside the shape to draw a little extra form but instead of adding... it creates a new shape.

https://ibb.co/f2GV2qr

I want to achieve the modification of the fourth image... but I just get 2 shapes (first and second image)

Any clue why is this happening? Thx for the attention.