Comment by danoprey

Technically, it actually works the other way around from vesting. Instead of you having 0% of your shares and working your way up to 100%, you start with 100% of your shares but the company has a Repurchase Right to buy back shares from you for (effectively) nothing. They incrementally lose this right until they have the right to repurchase 0% of your shares, which is when you are "fully vested".