Insiders generally only have certain times that they can trade stock. It's usually 3 weeks before the end of each quarter until the 2nd business day after the results are announced. But, if they have insider information about any material events, they they are also barred from trading.

To get around this, many insiders create a plan to automatically sell stock on a regular, typically quarterly basis. This is a practice accepted by the IRS, and as the number of shares are date of sale are set well in the future, the insider is typically safe from being accused of trading on insider knowledge.

The downside is that you are at the risk of market fluctuations, and sometimes you sell at $21/share...