There is no tax implication right away after 1y. It's only when they get converted to actual shares (which you can sell) you are taxed. In an IPO case that might be a few months after the IPO (still before the usual 6m IPO lockdown). I don't know how it works if RSUs are allowed to be vested for selling on secondary market.
The tax implications on a markdown after the first block of vesting completes is something I don't want to think about...