Actually, the key part is "to the extent such content does not embody the Product Identity." All that is necessary is to claim that your specific expression of a given mechanic is Product Identity. So long as the mechanic is something original (i.e. not derived from the SRD or another OGC source), it's now closed.
One example would be "Power Points" in Mutants & Masterminds, which are explicitly declared as PI in both the front of the book and in the OGL at the back. You can write your own definition of a Power Points mechanic (including one that was very much like PP in BRP), but using the M&M mechanic as written in an OGL product is verboten and, depending on how closely the rest of your product hewed to M&M, would very possibly get you in hot water with Green Ronin. There are other examples, but M&M is the easiest to use because GR is polite enough to make their PI declaration explicit (unlike, say, Monte Cook...).
KoOS