It seems to be working the wrong way from what you've described, raking aft should increase weather helm.

The only explanation I can come up with is that the further you rake the mast aft, the earlier your mainsheet blocks out, meaning less leech tension, meaning the centre of effort of the sail moves forward, which reduces weather helm.

However I'm not 100% familiar with the Nacra 14sqm and whether or not it can block out with lots of rake.