Hammer Tutorial - Phys_slideconstraint - Source

The Tutorial in text:

1. Crate a func_physbox/prop_physics and give it a name

2. Create a phys_slideconstraint and put it at the starting point were your'e brush/prop is going to slide

3. Drag the "middle-point-thing" in the mith of the phys_slideconstraint to the end point of your'e slide

4. Goto properties of the phys_slideconstraint

5. Set "entity 1" to the name of you're func_physbox/prop_physics

6. Goto flags and uncheck "no collision until break" and check "limit endpoints" if you want a limit of the slide

7. You are now done! :D