When setting up a SolidWorks Motion study, there are times when you may need to restrict the movement of parts in the assembly. Assembly Mates are the primary and intended method of constraining models in SolidWorks Motion.
If you are a SolidWorks Simulation user, you may be familiar with restraining components using Fixtures such as “Use Reference Geometry” to specify that components must not deform in particular directions within the Simulation study. This might lead you to believe that assigning a zero displacement Motor in SolidWorks Motion will achieve the same type of restraint. A zero displacement Motor will "restrain" movement of components, however it will not "prevent" motion entirely. If there are external forces acting on the components, then the Motors may not be sufficient to completely counteract those forces. Numerical errors in the solution may also cause a Motor to fail in completely preventing movement.
If you were to run a SolidWorks Motion study with zero displacement Motors and another without, you might see that the Motors restrain movement, but they may not completely eliminate movement.
Therefore, it is recommended to use assembly Mates rather than Motors if you would like to completely prevent movement in a particular direction in SolidWorks Motion.