Power Tapping (Aura) from Rory Fansler's Supernatural Energy in Pyramid 4/1 (pg 36)

[Refplace]

Power Tapping was meant to work for whatever source is appropriate in the campaign and fits the user. A psychic vampire could tap emotions, a fire elemental tap fire, etc. Word count forced me to trim some of the original material that would have made that clearer but I was really grateful to get all that I was able to in there.

Mages and spirits are typically different kinds of beings, though that depends on the setting.
You can only tap and transfer energy from and to a similar Energy Reserve so magic to magic or psi to psi.

Good callout on Improved Zombie, I had forgotten about them. However I arrived at the idea the same way, using the Affects Others modifier from GURPS Powers and would treat it that way. As it clearly states you need to use a Ready maneuver I felt no need to repeat that. Specific cases could vary but typically an apprentice or mage with that ability (ER with Affects Others) would take a Ready or optionally Concentrate to sort of chuck the energy to the receiving mage. However Power Tapping is an action taken by the mage who is drawing or pulling on the energy. In this case the subject would not need Affects Others on their ER.

I hope this answers your question and thank you for asking it and reading my article!