Hristo Velev visually explains how constraints work in Side FX Houdini.
Constraint networks might be a little challenging to figure out for new users in SideFX Houdini. Attempting to explain how they work on a low-level, is a new tutorial from Hristo Velev. The tutorial walks through a couple of scenarios for working with constraints.
You can see step by step what is happening both in the viewport and behind the scenes. Constraint networks are essential for more than just simulations in Houdini, you can also use them for complex rigging tasks. Have a look at how to extract twist rotations from a wrist bone, here.