In the APF you can let it sink in as far as you choose to.
Once you stop the downward motion you get the press command. At that point the bar is only allowed to move up.
You can expand your chest to give yourself some drive off of your chest (bar moves up from this).
But, after the command, you cannot dip the bar once more into the chest (downward movement) and then drive it up. That would be considered a "heave".