Complex sentence: Beginning in 2005, Byrne become known for his campaign against naked short selling, a practice which he says has been used in violation of securities law to hurt the price of his and other companies' stock.
Split into simpler sentences: Beginning in 2005, Byrne become known for his campaign against the practice of naked short selling. Byrne says it has been used in violation of securities law to hurt the price of his and other companies' stock.