It seems that Zynga is using a provision that says that all unvested stock grants are returned if the employee is fired. This is a pretty common provision from the contracts I have seen.
They then seem to be saying "give us back some of the stock voluntarily or we will fire you and you will lose it all."
If you want to make this impossible you can make your contracts such that unvested stock is not lost when one is fired. However, in that case you may end up with a lot of stockholders that are previously fired employees which may not be good for the company.
You can put in a provision in the contract that says that people can only be fired for good cause and that cause cannot be that they did not want to give their stock back. However, while this seems a very fair provision, it will invite a lot of lawsuits and may cost you a lot of money even if you do follow it.
You can put in a provision that says that one cannot ask employees to give back shares in exchange of not getting fired, but it is doubtful that will be effective. The courts usually allow contracts to be renegotiated if both parties agree to it, even if the provision that is to be renegotiated is the one that says "no renegotiation."
So, honestly, I cannot see a good way to "handcuff yourself" as you put it. You may as well just rely on being a decent person.
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PS: None of this is legal advice. This is just hypothetical discussion regarding a hypothetical situation. Please do consult a lawyer for your specific situation.
Employees can't rely on your decency because the investors might put someone else's finger on the trigger if that time comes. I wish there were a way to ensure everyone gets paid or screwed together, but tactics like share classes and dilution and liquidity preferences seem to be so strongly customary that I wonder if a VC would lose interest in a firm that relinquished the power to selectively starve that VC's rivals at the banquet.
"Unvested" means just that: The shares have not vested. The shares are not yours yet. You cannot lose something that is not yours. People who get fired before their shares vest, are not "losing" their unvested shares, nor are they "giving back" their unvested shares.
As mentioned, there's a very simple solution as mentioned below: vest monthly. IMO startup employees, especially engineers in this market, should start demanding it during negotiations -- it's a very simple way to defend yourself as an employee.
They then seem to be saying "give us back some of the stock voluntarily or we will fire you and you will lose it all."
If you want to make this impossible you can make your contracts such that unvested stock is not lost when one is fired. However, in that case you may end up with a lot of stockholders that are previously fired employees which may not be good for the company.
You can put in a provision in the contract that says that people can only be fired for good cause and that cause cannot be that they did not want to give their stock back. However, while this seems a very fair provision, it will invite a lot of lawsuits and may cost you a lot of money even if you do follow it.
You can put in a provision that says that one cannot ask employees to give back shares in exchange of not getting fired, but it is doubtful that will be effective. The courts usually allow contracts to be renegotiated if both parties agree to it, even if the provision that is to be renegotiated is the one that says "no renegotiation."
So, honestly, I cannot see a good way to "handcuff yourself" as you put it. You may as well just rely on being a decent person.
__________
PS: None of this is legal advice. This is just hypothetical discussion regarding a hypothetical situation. Please do consult a lawyer for your specific situation.