After further investigation I have discovered why the AMD resource monitor is flicking between 0 & 18%.
Modern GPU's downclock themselves when not under heavy load in order to use less power and prolong the life of the components. Slower clocks use less power, so they can supply a lower voltage, which generates less heat.
When running Amber it is using 18% of the downclocked core, or 0% of the high clocked core.
In the zip file is a csv file with the usage readings in if you care to look at an accurate set of readings over time.
But the simple answer is that Amber requires very little CPU or GPU.