For working around the fact that you can't easily "flip" or "mirror" physics bodies like RigidBody2D. It works via rotating and translating children of such nodes instead of mirroring them via scale(-1, 1).
As such it has several limitations, primarily that it only works for symmetrical children like a CollisionShape2D with a RectangleShape2D property, not one with an asymetrical hand drawn ConvexPolygonShape2D.
To use it, copy the files into your project (or better yet, git submodule add it), then change the extends RigidBody2D in your node's script to extends "subdir/FlippablePhysics2D.gd".FlippableRigidBody2D. See the example project in the godot-tricks repo.