Select the Rectangle Mask tool

Option-R

Select the Circle Mask tool

Option-C

Select the Freehand Mask tool

Option-P

Draw a mask proportionally

Shift-drag in the Canvas

Draw a mask from its center

Option-drag in the Canvas

Draw a mask proportionally from its center

Option-Shift-drag in the Canvas

Snap the rotation of a mask to 45-degree increments

Shift-drag a rotation handle