The (arguable) tax advantage is that the employee isn’t taxed until they are able to sell the RSUs for the same value they are taxed at (since company is public

The (arguable) tax advantage is that the employee isn’t taxed until they are able to sell the RSUs for the same value they are taxed at (since company is public and lockup is past) so you avoid a tax burden on a non-liquid asset. The “double” trigger is the ipo requirement plus the usual time-vesting for stock grants