Arm — tap empty space to drop a pin, tap again to close a straight rigid arm.
Ground — tap a pin to fix it (⏚).
Length — tap an arm to type its exact length; the mechanism re-solves to match.
Equal — tap 2+ arms to select them, then Apply — locks them to the same length as a group. Editing one member's length updates the rest.
Midpt — tap two pins/points; creates a new pin that always tracks their midpoint, even while dragging.
Test — drag any free pin to see it move.
Delete — tap an arm to remove it.
Rigid shapes — a square unit needs its 4 sides plus a diagonal brace to be rigid, otherwise it's a floppy 4-bar.