Enhance passes generated images through NovelAI Diffusion a second time to improve upon them, based on the prompt. It differs from upscaling in the way that your text prompt affects the enhancement as well, allowing you to adjust the prompt to, for example, focus the AI on any lacking aspects if needed.
The Magnitude slider uses combination value combinations of Strength & Noise. You can select the Show Individual Settings Option to set the individual values yourself.
If you want full control over the way your image is going to be affected then select Show Individual Settings. Now you can manually adjust both the Strength & Noise values as you see fit.
Setting high Strength values will change the image significantly. A low Strength affects the original generation less. A higher Noise setting may allow the AI to generate more detail.