Transmutation of Will

All magic involving a conscious user involves this idea. Fundamentally, it's the conversion of an individual's magical energy into another form. This energy exists within the soul, taking a form known as Will, hence the name: one transmutes their Will into another force. There's generally very little differentiation between upper and lower case will, as most cultures treat them as functionally the same thing. The soulsmiths of Whirlpool occasionally get pedantic and note that lower-w will is a mind-phenomenon, and upper-case Will is on the soul side of the Mind-Soul Interface.
What the Will is converted into varies greatly. For example, the hard-light projections of Arcanism transmute will to another Celestial energy, the dimensional shadow of which forms the physical construct.
Planar magics use a physical (generally biological) component which acts as an interface between will and Chaos phenomenon, since direct conversion is impossible due to the distance in reality layers.
The term 'transmutation of will' has its roots in Cotd Artifice.

Efficiency

A major bottleneck in any Will transmutation is perception- the more intuitive something feels, the more efficient it generally is. This is why mages tend to experience less soul draw when projecting more complex, but commonplace, objects than platonic solids, which should be much simpler computationally. The general ballpark of transmutation efficiency exists in the 0.1-1% range, though there's of course some muddiness when trying to quantify and apply physical concepts to energy which inherently stems from a subjective reality.

Physical draw (soul transmutation)

Transmutation of Will refers to the process in which Will is converted to other energies. Generally speaking, the Will exists in raw form within the soul vessel, and the maximum rate that one can draw upon this is known as their soul flow rate.
However, this isn't always the case. Magic can also be cast using physical energy- a somewhat nebulous concept. In doing so, the physical energy is first converted into will, before the standard transmutation of will into something else takes place. This process is known as soul transmutation, or transmutation into Will.
Quantifying physical energy can be difficult- borderline impossible, even. A decent approximation of it up to a certain point would be to think of 'physical energy' as the body's ability to generate force. Drawing physical energy causes strain and damage to one's muscles just as if they were in use. Which muscles are drawn from, and the ratios of usage generally depend on intent.
Past the point of muscle-use, physical draw begins tapping into fat stores and then muscle in something much resembling starvation.
The problem with any sort of formal quantification is the inherent inconsistency found within the soul. The concept of 'effort' is based within the Celestial plane and therefore bound to the subjective laws involved. While most acts are still bound to one's biology (adjusted to an extent by flubbing), transmutation of Will occurs in the soul and not the brain- as a result, there's a theoretically infinite limit on 'effort', which skews results heavily.
Unlike ordinary exertion, soul transmutation, for whatever reason, doesn't work like training despite the fact that it exhibits all the same physical symptoms. This is hypothesized to be because the damage happens on the soul-end, so flubbing heals the micro-tears etc. that form, as opposed to biological processes (which would ordinarily result in the building of muscle).
Of course, the theory of micro-tears is in itself semi-debatable anyway, and this doesn't explain why soul transmutation doesn't improve cardiovascular health.