Inside the Mind explains all the Houdini RBD Constraints and how they work.

Rigid Body Dynamic Constraints, or RBD constraints, are essential to making RBD simulations do what you want them to.

This new tutorial from Inside The Mind covers how constraints work in Houdini and how to manipulate them inside the solver using Wrangles.

Also, check out this tutorial from Rohan Dalvi shows how you can help guide rigid body simulations in Side FX Houdini using the Guide Geometry Node if you want even more control over RBD sims.