Opening your throat is not just about relaxation. There is more you need to do to give your vocal folds as much room as possible. Beside giving room to the vocal folds you will also increase resonance by widening your throat.
One of the best ways to give your voice room is SMILE!
An inner smile makes your throat wider, if you exagerate an inner smile you can feel certain muscles thicken. By the way, this particular muscle activity is NOT bad!
When you open your throat like this, you open your false vocal folds. By doing that you allow your true vocal folds (the ones you sing with) to move more freely.
Your breathing becomes more effective too.
When you practice this widening of the throat it will help protect your vocal folds from damaging.
Another way to open your throat is thinking about a happy surprise. You will feel your throat widen. While singing, keep it that way.
The higher the notes you sing, the more important the open throat becomes. So increase your inner smile while ascending.
Many pop, rock, and gospel singers use the opposite of a rectracted throat; a constricted throat.
Although it adds some interesting colors to your vocal palet, constriction of the throat is one of the main causes of vocal trauma, so use it only when a good voice teacher showed you how it is done!
A nice article about retraction can be found via this link.