Challenge 1: Why the Selection Is Inverted Before Masking the Blur — Solution Walkthrough

Why inversion is needed:

The Step 2 selection covers the subject specifically. The goal in Step
4 is to blur the background while keeping the subject sharp — meaning
the blur effect needs to be hidden exactly where the subject is, and
visible everywhere else. Per Chapter 5's own black-conceals-white-
reveals rule, using the subject selection directly (without inverting
it) would make the mask show white over the subject and black over
the background — the exact opposite of what's needed, since that
would reveal the blur over the subject and hide it over the
background.

What inverting actually accomplishes:

Per Chapter 3's own Select Inverse/Invert material, inverting flips
the selection to its exact opposite — now everything EXCEPT the
subject is selected. When this inverted selection becomes the mask,
the background area (now selected/white) reveals the blur, and the
subject area (now unselected/black) hides it — precisely the intended
result.

Why this ties two earlier chapters together directly:

This step is a concrete, combined application of Chapter 3's own
selection-inversion material and Chapter 5's own mask-from-selection
material — neither concept alone would produce the correct result
without the other.

WHY THIS WORKS AS AN ANSWER
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This exercise checks that the specific reason for inverting the
selection is understood mechanically — tracing through exactly what
would go wrong without the inversion, not just recalling that
"inverting was needed" as an isolated fact.