Basically any tube will have a plane which it ''likes to'' bend on
Building on this, the spine as it's called, will improve a rods performance no end, mainly casting accuracy thing but also the feel is better and far less twist
I don't accept non spine built rods any more
Most of the big manufacturters will rubbish the importance of it for a simple reason - they can't be bothered to do it
Daiwa's upper range of rods is built in the UK, on the spine. They are the only big manufacturer which bothers. You're looking at Whiskers and above. I have a pair of Whiskers as my spare rods and they are very good