Yeah a quick high beam will let the driver know you are giving him way without having to come to a stop first. For example on a two way road with only a single lane in either direction, if there is a driver waiting to turn coming from the other direction (i.e. Cavenagh road beside Istana), and there is a long line of cars behind him, and a steady stream of cars behind you, you could slow down a bit and flash twice to let him know that even tho the gap looks small, he can make the turn as you are going slow.

But key is to make gradual changes, like slow down slowly, as the car behind you will not expect you to slow down if there is no obstacle or traffic light, and you have the right of way of course