The logic for "ignore" can be confusing. It's best to think of it as a filter that excludes things. Or as an "if not" statement.

Since FontLab uses the AFDKO, the Adobe site is a good resource (IIRC, the FontLab manual does mention a few deviations). Your ignore line can be found here:

Specifying exceptions to the Chain Sub rule

The most confusing part of it is it also needs a at least one part tagged with a ' (in regular substitutions this marks the to-be-replaced glyph(s)).

Speaking of those 'ignore sub' statements: Within any lookup in a features file (which often means within a feature, unless you've specified lookups within a feature), scanning stops at the first match. So, in OpenType layout, if 'sub f i by f_i' matches, then that substitution is made, and everything in the lookup after that is ignored/skipped. The 'ignore sub' statements function exactly like any other 'sub' statement in this regard, except that instead of performing a substitution, nothing is done.

So, the statement 'ignore sub x f' i' by f_i' can be read as "if f and i is preceded by x, do nothing." This is a match, like everything else, and subsequent lines in the lookup are skipped.

Don't know if that actually helps! :)

While we're on the topic, anyone know how to get at OpenType features in Pages?

(Mac, iWork)

Sure: Select the text, open the Fonts panel (command-T), and choose "Typography..." from the the pop-up menu at the bottom with the gear icon. If the text is set in an OT font, the features will show up there.