Here is the interaction between cowl flaps and speed brakes on an airplane that has both - When you want to descend fast you close the cowl flaps to keep the engine from overcooling and you extend the speed brakes to increase your drag. If you have ever seen and engine stop from being too cold (think winter extended glides at idle power) then you will want to keep your engine temps well into the green arcs on descents.

The two controls are used almost always oppositely at the same time.

Best of luck,

Wes