This just popped up seemingly randomly on my HuWz.
The ability to tap ambient magical fields is granted by worshipping Sif Muna. Like all abilities, it is activated via pressing the a button and then selecting the ability. When activated, you instantly regain a small amount of mana, but you become more hungry. It is mainly used when you are out of mana, but are in a tight situation and cannot spend any time resting to regain it.
The CrawlWiki has all the gory details. The formula for the actual amount of food lost doesn't help much during a game, since you never know your precise food level. However, the important information is that your Invocations skill and Intelligence stat control how likely the ability is to succeed, while just your Invocations skill controls how much mana you gain.