Metal in F — alternating bass fingerstyle
A power chord has only one bass string (the root) under its 2 treble strings: unlike a full chord, where the thumb can alternate between two different bass strings without changing chord, the only real alternation possible here is the one that comes with the chord change itself, from F5's root to C5's. The term 'alternating bass' therefore applies here a bit differently than on the catalogue's full-chord grids.
Tips
Don't look for a bass alternation within a single held chord: on a power chord, there's only one bass string available.
Focus instead on the thumb's regularity right at the moment of the chord change, as it switches from F5's root to C5's.
Compare with one of the catalogue's full-chord grids (like the blues or pop progressions): there, a real bass alternation exists even on a held chord, with no change.