What is Behind Section 301 Investigation into China?

On August 14 US President Donald Trump issued a presidential memo directing the United States Trade Representative (USTR) to conduct a Section 301 investigation into China. Robert Lighthizer, the USTR, subsequently announced the process.
Trump explained the reason behind the move as China’s “forced technology transfer” and “theft of American IP”, and called the investigation “a very big move” and “just a beginning.”