Hey all, I wrote a blog about this tool (https://www.artstation.com/thomasfraser/blog) for Maya. I've told a bunch of people about how it can speed up their modeling but it's falling upon deaf ears. Use Constraints is extremely handy when you want to choose edges, verts, or faces which have specific properties.
Here is an image of a simple model I've created. Whether I want to bevel all the hard edges, or cut them for my UV mapping, I'd like to select all of them in just a few button clicks, instead of manually selecting each hard edge/edge loop one by one. To do this, open Use Constraints (Select > Use Constraints) and you'll see the below window pop up. I have this docked as I use it frequently.
Right click on your model, and select 'Edges', and you'll notice your Use Constraints menu might change. The menus for constraining for verts, edges and faces are all different. Next, click on "All and Next", and "Hard," and all of your mesh's hard edges will automatically be selected. This of course is extremely handy the more complex your mesh becomes.
Sometimes my model is an ugly mix of hard edges which should be soft, or soft edges which should be hard. Below, I've just softened the entire model instead of going in and fixing each incorrect edge one by one. But I would like all 90 degree corners to be hard!
You're going to need to turn off the Properties - Smoothing selection so as you can select all edges. Open the 'Angle' menu, and click on 'Activate'. Here, play around with the minimum angle requirement, and bringing it to 90 degrees will select every 90 degree angle on your mesh! Now I can simply harden or bevel these edges, and it only took a few clicks.
I'll point out that sometimes I've left other constraints on, such as trying to constrain using an angle, but also had the constraint for hard edges activated, and it has caused frustration a lot. If Use Constraints is not working properly, scroll through your menus to see if you have any other constraints activated which you can turn off.
One of the more useful situations to use this tool in is after decimating a mesh from ZBrush (as below), as the mesh is now triangulated, and selecting edge loops is now impossible. It is a complicated mesh for sure, and I'd like to cut the UV up based on the cavities of the mesh. After playing around with the angle settings, I've been able to find an OK setting for my constraints. I will, however, need to spend a little bit of time cleaning the cuts up.
If you have any tips and suggestions, I'd love to hear them!