Braking is not gradual on release in this case: when the driver puts the controller on "release" position, brakes are fully released; if he brakes again too shortly after, there is the risk to have a loss of braking power because there may not be enough air to brake optimally, since the absence of the fixed pressure of the command reservoir doesn't ensure that there is enough air to safely release the brakes.
In this specific situation, braking is abrupt upon switching to the "release" mode, as it completely disengages the brakes. If the driver presses the brake pedal again shortly after, they may experience a loss of braking capability due to inadequate airflow, which can compromise safe operation of the vehicle.