Tesla Motors, which had its IPO about a half a year ago, has been rocketing higher for months burning short sellers along the way. Yesterday, it fell 15% as the lockup to sell shares expired. When a company goes public typically only a small portion of the company starts trading while the remaining shares held by company insiders or outside investors are locked up and cannot be sold until the lockup expiration.
Lockup expirations were very important during the Internet bubble. Many companies losing money would have market caps in the tens of billions of dollars while less than 20% of their shares were trading. Suddenly the lockup expiration would hit and the stock would melt as a wave of sellers hit the market.
I stopped paying attention to lockup expirations a long time ago because they only tend to matter with very overvalued companies. However, with all these China IPOs that few know anything about (replace .com with China) it could start to matter again. If you hold a recent China IPO highflying stock it might make sense to pay attention to the lockup expiration. There is a free calendar each week in the Wall Street Journal.