What is waterblock bow?

[etmorgs42]

I've been reading a lot about waterblocks for i7s and a lot of the stuff I've read mentioned blocks that have a bow or that you tighten blocks to give it a bow.
What does this mean and how does it affect the performance of a waterblock (better or worse)?
Also how is it pronounced i.e. "bow" as in the bow of a ship or "bow" as in bow and arrow?

Maybe it's a simple question but I feel like it might be important.

 

[rennyf77]

the top of the i7 heat spreader isn't flat, hence a block that accommodates that curvature might deliver better temps.

 

[etmorgs42]

Okay thanks that makes sense.