Creating a mask

When you apply the Draw Mask (B), Linear Gradient Mask (L), the Radial Gradient Mask (T), the Healing Mask (Q), or the Clone Mask (S) to an image, the Layers 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.