Drakeman, the .390 is the amount of deflection or flex in that shaft with the shaft supported on both ends and a certain weight hung in the middle. The lower the number, the less flex, meaning the more stiff the arrow. A .340 is more stiff than a .390. A 390 is more stiff than a .400.

So, if you want a stiffer arrow than a .390, look at the charts and choose one that is lower than .390.

If you want an arrow with less spine than .390 you find one with a higher number.

Easton has an arrow spine and weight comparison chart. Link to it is on their home page. Not much good for anything, unless you're trying to find a lighter arrow in close to the same spine range, but it's a good thing to look at, to get an idea how this stuff works.