It is bit hidden in the rules. The rules for movement are:
Each time you move a unit, you can move any of its models you choose to. The controlling player chooses the order in which to move their models. Whenever you move a model, you can pivot it and/or change its position on the battlefield along any path, but no part of its base can be moved across an enemy model or cross the edge of the battlefield. It can be moved over friendly models as if they were not there if you wish, but it cannot end its move on top of another model. The only exception to this is when moving Monster or Vehicle models; such models cannot be moved over other friendly Monster or Vehicle models and must be moved around them instead. The distance a model moves is measured using the part of its base that moves furthest along its path. If a model does not have a base, measure using whichever part of that model moves the furthest.
The Rules Commentary adds:
Moving Over Models: Some models have a rule that enables them to ‘move over models’, or ‘move over models as if they were not there’ when making a move. When moving a model with such a rule, it can be moved within Engagement Range of enemy models, but it cannot end a move on top of another model, it cannot end a Normal, Advance or Fall Back move within Engagement Range of any enemy models, and it can only end a Charge move within Engagement Range of units it declared a charge against that phase.
Given this, you Move Over another model when part of the moving model that you measure from crosses part of model you measure from on the other model. Essentially, the distance between the models would have to be less than 0".