This comes in handy when you want to scribble over your presentation to explain something to your audience.
P to change your mouse arrow into a red pen and start writing over your slide.
To erase whatever you scribbled, just press
To return back to your mouse arrow, just press
At the end of your presentation, PowerPoint will ask whether you want to save the ink annotations you made during your presentation. You can either Keep them or Discard.