Creating a mask

When you apply Disegna Maschera (B), Disegna Maschera Gradiente Lineare (L), Disegna Maschera Gradiente Radiale (T), Disegna Maschera di Correzione (Q), or Disegna Machera Clonazione (S) to an image, the Livelli tool automatically creates a mask along with an adjustment layer. You can then make your localized image adjustments.

If you have created an empty layer first, you apply the desired image adjustments and see them being applied as you draw the mask.