Defending Position

[Paul Swire]

You can utilise an aerodynamic ‘tow’ from the car in front. This is achieved by moving into an opponent’s slipstream – a pocket of low pressure air behind a car through which the following driver can move more freely and gain a small speed advantage. Allowing a faster approach to the next corner.

Great post but this applies more to attacking/defending a single corner. At Brianza we drive straight for a great deal of the lap and it's then, when weaving around or zigzagging, should not be allowed.
The above article applies if you act like that the whole straight, as in the front stretch, the backstretch and the straights between the banking.

Mentioned in the paragraph above.

A single maneuver is allowed to defend your position but moving side to side (blocking) is not allowed