Magi of Hermes question

I have begun going through my new copy of Magi of Hermes. Something puzzles me in the first character's description, Alexander.

Several of his described spells are human transformations into different animals. Many include a statement that he carries an item made of that particular kind of animal's hide. They are alternately described as "aphysical focus" or "aphysical prop", and they allow him to cancel the (sun-duration) spell at will.
Now, he neither has Harnessed magic virtue nor Harnessed Casting mastery abilities, and some of his other (non-transformation) spells specifically state that he is unable to cancel his spells at will.

What am I missing here? What rules do these "physical focuses" refer to?

Thanks for any help,

Muto Corpus spells with a Personal range can be ended early by concentrating and using a physical focus (See the penultimate paragraph of "Muto Corpus Guidelines" on page 132 of the Core Book.

This makes shapechanging spells more practical because the caster isn't trapped in animal form until the spell expires. The in-game justification (as stated in HoH: TL) is that most of the Order's shapechanging spells were designed by a Mercere Magus with the Mutantum Magic Virtue which allows Boosting, Harnessing or Tethering of spells. Experimentation on his part when inventing those spells resulted in them having a "Harnessed" side-effect...