Complex sentence: Normal spellers have access to a lexical spelling system that uses a whole-word; when functioning properly, it allows for recall of the spelling of a complete word, not as individual letters or sounds.
Split into simpler sentences: Normal spellers have access to a system that uses a whole-word retrieval process; this system is called the lexical spelling system. This system, when functioning properly, allows for people to recall the spelling of a complete word, not as individual letters or sounds.