Entity Name: prop_dynamic
A prop_dynamic is an entity that creates a model wherever it is centered at. The prop_dynamic entity can also be told to play animations. Common uses for this entity are for levers and buttons. Prop_dynamic entities can also have their skin changed. An example of this is the keypads found in abundance in Dr. Eli's lab.
Another common use for this entity is to make models that cannot be used as prop_physics.
This entity cannot move on it's own. It can be parented to a moving object (such as a func_physbox).
So if you wanted to make the butcher knife in props_lab into a knife you can actually pick up, you can create it as a prop_dynamic entity and parent it to an invisible func_physbox.
Modelled Buttons can be used by covering a prop_dynamic button with a func_button, and then targeting the prop_dynamic and telling it to SetAnimation.
Levers can be used by covering a prop_dynamic with a func_button_rot. Parent the prop_dynamic to the func_button_rot.
Valves can be created by using a prop_dynamic with a valve model and covering the prop_dynamic with momentary_rot_button. Parent the prop_dynamic to the momentary_rot_button.