Use Inertial Relief
If a model is not adequately restrained, a small force can cause rigid
body motion. Even in cases when the applied forces are balanced, a small
unbalanced force can result from numerical approximations.
When this option is checked, the program automatically applies forces
to counteract unbalanced external loading. This option is particularly
useful when you import loads from SolidWorks Motion where external loads can
be slightly unbalanced. When you check this option, the program does not
complain if the restraints are not adequate.
Do not check this flag if the external loads
are not approximately balanced.