Tap the wrist guard icon in the active tool's ribbon (upper right of your screen) to activate this feature:
Use the tab at the top right of the wrist guard to adjust the size of the overlay as needed. You can then rest your wrist or palm on the bottom portion of your screen while annotating at the top:
The wrist guard will stay open in the same position (on a document-specific basis) while using any of the tools mentioned above until deactivated.
You can, however, set it to only stay open for the Pencil tool via App Settings > Annotation > "Auto-Display Wrist Guard for Pencil Only."
Please note: The wrist guard cannot be moved to the top of the screen at this time (sorry lefties!).