FIN28 is one of two attribution methods for expense scenarios under the GAAP standard, the other being the graded straight line approach. It was first introduced in FASB Interpretation No. 28 and is sometimes referred to as the accelerated manner, 'front loading' the expense, or the graded attribution.
Under the IFRS accounting standard, expense for service based equity grants should be recognized using FIN28.
Each vesting increment is treated as its separate award each with its own service period that all begins on the grant date and is amortized concurrently. The theory that the stakeholder is providing service to earn each vesting tranche within the award simultaneously.
Here's a post that compares the FIN28 method to a straight line method.