Find a diagram of the car's resuspension set up; the tops of the uprights are tied together by the car's chassis, while the bottoms are not. It isn't the chassis that is flexing, it is the uprights themselves that are moving. Under hard turn in, the inside strut is pulled outwards while the outside upright is pushed inwards because neither is really firmly attached to anything substantial. Tying one to the other helps reduce the deflection impact on both struts, and slightly preloading the tie bar (read tightening) increases that load sharing and reduces deflection, and the resulting tire scrubbing, for both uprights.
Ideally, you would run a triangulation bar upwards on a 45 degree angle from the bottom of the uprights toward the centerline of the car, where they would connect to a bar running across from one side of the car to the other, which would prevent movement in any direction; only problem is that the triangulation bars with have to run through the transmission, which happens to be in the way. So going across underneath is the next best alternative.
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