A few technical points:

1. U.S. tax law requires that incentive stock options (ISOs) have a 90-day termination tail on them - that is, the options do not qualify as ISOs if they are not issued under an enforceable agreement by which all vested options must be exercised within 90 days of termination of employment or expire.

2. As a direct result of point 1, the near-universal practice that startups use when issuing stock options is to require employees to sign documentation by which they agree that they lose even their vested options if they do not exercise them within 90 days of their termination date.

3. The original idea behind ISOs (the reason for distinguishing them from "non-statutory" or "non-qualified" options) is that they can be exercised at any time after they vest without triggering ordinary income tax for the exercising employee at the time of exercise (subject to certain monetary limits).

4. However, the "benefit" note in item 3 above, though a very real benefit in days gone by, is often illusory today because the exercise of ISOs requires that the amount of the spread (difference between exercise price and fair market value of the stock at the time of exercise) be included as an item of AMT - meaning that a so-called "ISO" exercise can result in a huge AMT tax. What this means is that an employee holding vested options in a startup whose value has risen greatly faces a dilemma. "Do I exercise now, pay a huge tax, hold only a piece of paper that may or may not be worth anything down the road, and hope that the stock price holds? Or do I wait until a liquidation event, exercise at that time, and thus eliminate the large tax risk of exercising before that time?"

5. Of course, many such employees chose to wait. If you cannot sell your stock right away, it is a very high risk to pay a massive tax on an option exercise only to wind up holding illiquid stock that may become valueless if things don't go right. This is no small issue. People committed suicide during the dot-bust era a decade or so ago after having made the wrong decision.

6. The problem is exacerbated by the 90-day expiration rule on employee termination. It is one thing to wait and wait until the liquidation event. But what if it is a long time in coming? Many employees are trapped in their employment when they face the above dilemma because leaving their employment will force them to exercise or lose potentially valuable options if they don't exercise within 90 days of termination. Even worse, if their employment is terminated by the company, they are put in the horrible position of losing something they worked very hard for because of a seemingly arbitrary and stupid rule saying that the options expire within 90 days of termination.

7. Well, guess what, the rule is stupid and arbitrary in many ways. Why? Because, though the law requires that it be made part of any ISO grant, this does not mean that the startup cannot modify the agreement for the benefit of the employee to eliminate the 90-day expiration provision and to give the employee room to exercise for an extended period. What happens when the company does this in spite of the tax rules? Well, it means that the option loses its character as an ISO. But it does not lose its character as a legally enforceable stock option. It just means that it is taxed as an NQO (non-qualified option). So, in a worst case, the employee exercising an NQO gets immediately taxed on the spread at the time of exercise. But, if that modification gives the employee the flexibility to carry the options unexercised but exercisable for a period going well beyond the 90-day period currently defining expiration of such options, the employee can leave employment, hold the options, and wait for such extended period (e.g., the 7 years discussed in the article here) before deciding whether to take the tax risk of exercising and holding illiquid shares. And, if, in the interim, the company does have a liquidation event, then the employee's dilemma vanishes and he can exercise and sell right away in order to make the profit and pay the associated taxes.

8. The net result of the startup's agreeing to eliminate the 90-day provision is that employees need no longer feel trapped in their employment once their options have vested when the stock price becomes high. They can leave and still keep the option of cashing in on their hard-earned options.

9. So why don't most companies agree to such modifications? Their lawyers will tell you that the law requires the 90-day rule and this is true but misleading because that requirement can be violated in a lawful way that simply changes the character of the options. Thus, if the parties can live with the change, why not do it routinely? Well, I leave it to everyone to draw his own conclusions. Perhaps startups want to keep their employees trapped. Perhaps they are just listening to lawyers who insist on following the rules. Or perhaps a board of directors does not feel it consistent with fiduciary duties to give employees something that appears to be a gratuitous modification that does not benefit the company. Whatever the reason, it remains true that most startups choose to leave their employees trapped in this dilemma without even giving them a good explanation beyond the fact they agreed to the condition and that the the tax laws "require" it.

In any case, kudos to Pinterest for doing the decent thing here. It is indeed all too rare to see something like this and they are to be congratulated for it.