From Twitter's Blog (emphasis mine):
The IPA is a new way to do patent assignment that keeps control in the hands of engineers and designers. It is a commitment from Twitter to our employees that patents can only be used for defensive purposes. We will not use the patents from employees’ inventions in offensive litigation without their permission. What’s more, this control flows with the patents, so if we sold them to others, they could only use them as the inventor intended.
This is a significant departure from the current state of affairs in the industry. Typically, engineers and designers sign an agreement with their company that irrevocably gives that company any patents filed related to the employee’s work. The company then has control over the patents and can use them however they want, which may include selling them to others who can also use them however they want. With the IPA, employees can be assured that their patents will be used only as a shield rather than as a weapon.