A free body diagram is the first thing a mechanics course teaches and the last thing a student wants to draw. Physion can now draw one for you, from the real forces in a running simulation: select a body, switch on the forces you care about, and each one appears as a labelled arrow, live, while the scene runs.
The arrows are measurements, not illustrations. Their magnitudes come out of the same calculation that puts the block where it is, so they are exactly as true as the motion you are already watching, and they change the instant anything else does. That turns a simulation from something to watch into something to read: a block sitting still on a ramp is not much of a lesson, but the three forces holding it there, labelled and adding up to nothing, is most of one.
The first half of this post covers what the feature does and every option it has. The second builds a real experiment with it, ready to take into a lesson.