Taxed as income based off of the fair market value of the RSUs as determined by a independent auditor's assessment (It's called a 409A Valuation). In practice

Taxed as income based off of the fair market value of the RSUs as determined by a independent auditor's assessment (It's called a 409A Valuation). In practice most companies will allow you to surrender non-liquid RSUs to pay the tax withholding so you don't have a massive tax bill from a non-liquid asset.