Okay...I've been reading through this a few times, and I think I get it.
All Trigger effects "trigger," which means they get the chance to activate. For example, if Sangan and Witch of the Black Forest are both destroyed by Dark Hole, they all "trigger" and form a chain according to SECOG.
Optional Trigger effects are the same, except (1) their requirement for them to "trigger" has to be the last thing to happen and (2) you may chose to not activate them, and they won't be part of the SECOG.
However, for Theinen the Great Sphinx, you have the option of "triggering" his effect. If something happens immediately after the Sphinges are destroyed (i.e. you summon a different Theinen), you miss the chance to "trigger" his effect, and can't summon him.
...Right?