I think the physics engine, and how the levels are built will ahve something todo with this trick, for example NExon has Invisible walls and Height restrictions, while Source can have user created levels without height restrictions Using Garry's MOD
This is not a map build by the ones who made counter strike source i think.
They have been doing trikz for many years and its what they do
And if you watch trough the movie they are doing shit on assult