You can either dispense more line automatically by pressing the trigger and releasing it for at least two seconds and it will automatically advance at least to the full length, or you can press the line advance button and release it and it will advance about 1/2 inch per press and release cycle, providing that you are pulling out on the string at the same time as you perform a press and release cycle. (Obviously, the manual press and release has to be done when the head is not rotating.)
I found with my three trimmers (one new, two used) that the automatic feed went through a lot of line quickly. The two used trimmers actually spun out a spool in about five minutes of operation. As a result, I replaced the spring in the head underneath the spool with a standard click-type ball-point pen spring. This disables the auto line feed so that I use a lot less line. I learned that I have to either listen to the pitch of the weed whip to hear when to advance line or watch how close the head has to approach the weeds/grass being trimmed and advance line when the head is getting too close or I have to take the spool retainer off if the line breaks off too short. If you run your trimmer constant like to trim a small lawn, this may not be the option for you, but if you start and stop the trimmer a lot (lots of trees or posts), this really saves a lot of line.
Compared to a Black and Decker string trimmer (my wife's parents own one that continuously feeds and they are a challenge to attempt any repairs due to the centrifugal action of the auto line feed), the auto feed function works much better on these trimmers, and it has the bonus of being easily disabled and re-enabled as desired.
Finally, remember that the cutting action of all string trimmers occurs at the outer edge of the rotating string - just like a circular saw. For longest string life, do not hold the trimmer head closely against a tree or a wall. You will just cause the string to wear faster and can girdle a young tree by doing this. Instead, hold the trimmer and approach the tree or wall smoothly until you can see that you are barely making contact.