Activates the entity. This needs to be used on some entities (like constraints) after being spawned.
Calling this method after Entity:SetModelScale will recreate a new scaled
SOLID_VPHYSICS PhysObj on scripted entities. This can be a problem if you made a properly scaled PhysObj of another kind (using Entity:PhysicsInitSphere for instance) or if you edited the PhysObj's properties. This is especially the behavior of the Sandbox spawn menu.