Move the slider to change Slant. The example letters below should shift to a "single story" version when Slant is greater than -7.5.
As of Nov 1, 2019, this test fails in:
ag
a
a
(Obviously, this shouldn't be necessary, but is a useful hack until this bug is fixed in production Safari on iOS & macOS.
Glyph substitution (GSUB) in variable fonts occurs when a character's shape is changed in a non-compatible way. Put simply, GSUB is when shape changes dramatically, rather than gradually, along an axis.
One example of this is the "Q" in Skia: its form simplifies in the bolder weights to retain readability.
For a full description of this, see the OpenType spec: tag: rvrn.