Complex sentence: Perform proofreading "in addition" to spellchecking, as spellcheckers will not flag transposed letters which result in an correctly spelled but unintended word.
Split into simpler sentences: Perform proofreading "in addition" to spellchecking. As helpful as they are, spellchecking programs usually will "not" flag transposed letters which result in a correctly-spelled but unintended word.