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 scenario, braking is abrupt upon switching to the "release" mode, as it fully disengages the brakes. If the driver presses the brake pedal again shortly thereafter, they may experience a loss of braking capability due to inadequate airflow, as the absence of constant pressure from the command reservoir cannot ensure sufficient air supply for safe brake release.