Great Weapon Master bonus damage

[Mr Adventurer]

When do you decide to apply the -5 to hit and +10 damage in relation to your attacks?

Is it whenever you make an attack, you can decide to do that or not?

Or is it once on your turn for the duration of your turn or something like that? (And if so, what about out-of-turn attacks?)

[Kryx]

Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack’s damage.

Before each attack.