Box Line Text (BLT) is a simple virtual whiteboarding tool. It is minimalistic and contains no visible UI for a clean screencasting experience.
Click anywhere to create a text element
Click and drag a straight line to make a line
Click and drag a diagonal line to make a box
Hold shift to use the mouse as an eraser
Press Ctrl-z to undo creating elements
This tool generally assumes that you are screencasting from a laptop or desktop, so it is optimized for using a mouse to create figures and a physical keyboard to create text. It has limited touch and pen support. There are much better tools available for that type of interface.