Complex sentence: Another theory of Holland's is regarding the timing of the first shot; he posits that the first shot occurred before Zapruder began filming; if correct, then this would affect numerous aspects of the traditional version of the chain of events.
Split into simpler sentences: Another theory of Holland's is regarding the timing of the first shot. One notable theory posits that the first shot occurred before Zapruder began filming; if correct, then this would affect numerous aspects of the traditional version of the chain of events.