Press Shift+Cmd+D to start annotating. Draw with the pointer. Press it again to stop and put the overlay away.

The tools

Tool	Key	What it does
Pen	`P`	Freehand, follows the pointer
Line	`L`	Straight line, drag from one end to the other
Arrow	`A`	Like the line, with a head at the end
Rectangle	`R`	Drag out a box
Ellipse	`E`	Drag out a circle or oval
Highlighter	`H`	Thick and see-through, for covering words
Text	`T`	Click where you want it, type, press `Esc`
Select	`V`	Pick out something you already drew

Hold Shift while dragging a shape to constrain it: a square instead of a rectangle, a circle instead of an oval, a line at a clean angle. Hold Option to draw out from the centre instead of from the corner.

Colours and thickness

Keys 1 to 5 pick the five colours. [ and ] step between the stroke width presets, three of them to begin with: thin, medium and thick. You can change the preset points, and the five colours, in Settings.

Tidying up wobbly shapes

Turn on shape beautification in Settings and a freehand pen stroke that was meant to be a circle, a rectangle or a straight line is swapped for a clean one the moment you lift the pointer.

It is off by default, deliberately: getting a tidy shape you did not ask for in the middle of an explanation costs you the stroke you actually drew. If it does surprise you, undo once. That gives the original back exactly as you drew it.

It applies to the pen only. The shape tools already draw exact geometry, and the highlighter is supposed to look hand-drawn.

The beautify tolerance slider next to the switch decides how forgiving it is: how far your stroke may wander from the perfect shape and still be replaced. Higher is more eager. It is measured relative to the size of the stroke itself, so a small doodle and a big sweep are judged by the same standard of sloppiness rather than the same number of pixels.

Clicking through to the app underneath

Press Shift+Cmd+A. Your drawings stay on screen, but clicks now reach whatever is behind them, so you can keep using the app you are presenting. The pointer goes back to the ordinary arrow so you can tell at a glance which mode you are in. Press it again to draw some more.

Moving and changing what you drew

Press V for the select tool, then:

Click something to select it. A dashed outline shows what you picked.
Drag it to move it. Undo puts it back where it was.
Double-click text to edit it. The words you typed come back and you can keep typing.
Delete removes what you selected. Cmd+Z brings it back.
Click empty space to deselect.

Erasing

Delete removes just the one thing you have selected. Press V, click it, then Delete. Everything else stays where it is.
Cmd+Z undoes, as many times as you like.
Shift+Cmd+Z redoes.
Cmd+Delete clears everything on the current display.
Right-click wipes the screen immediately.

You can also have drawings fade by themselves. See Auto-erase in Settings. Each stroke fades from the moment you drew it, so the screen tidies itself while you talk.

What is not here yet

The eraser tool has a key (X) and selects, but does not rub anything out yet. To remove something, select it with V and press Delete, or use undo or clear.

Annotating