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. It is even possible to fully exhaust the reservoir, with the consequent danger to not be able to stop the train at all. This is prevented in units with Breda and other types of valves as the brakes cannot be fully released if there is not enough air in main reservoir tanks to brake again.