What do you mean "when the capital gains" happens? Say I was an early employee of a company and received stock options. I exercised my options while living with

What do you mean "when the capital gains" happens? Say I was an early employee of a company and received stock options. I exercised my options while living within the US and have private stock. The company IPOs, I move to Puerto Rico, and sell my shares while a resident there. The "tax event" happens when I sell my shares, no?