Lets now hoist the jib and consider the result.
The jib now acts as the main did before – its the first sail to hit the airflow, and bend it around.
The main is now in the back-wash of the jib, and this creates an interaction between the two sails.
There is some arguments between aerodynamic experts and sailors about happens in the slot, with many theories, but the mathematical models show a complex and interactive solution.
This, the “circulation” model, demonstrates that the jib puts the mainsail in a header (it needs sheeted on more than if it were on its own) and the mainsail puts the jib in a huge lift (the jib can be more eased, or the boat sailed higher). The combination of these effects is much higher than each sail separately, so it is really important to have this happen.
It can take between 8 and 20 seconds to get this circulation to happen, and explains why it can be so difficult to get the boat moving again after a bad tack, or a big wave.
If the jib is sheeted to close to the main, the main back winds, so either the main is brought closer to the centreline, or the jib eased.
It is the main being closer to the centreline that is the best solution going to windward.
Backwinding of the main is considered to be acceptable, and a little is a good thing.