As I was thinking about the process of stage separation, I realized the empty stage dropped, often many tons, is usually separated quite gently, moving at sluggish couple of m/s or less away from the upper stage. The immediate thought was "What a waste! Even ejected at moderate speed, so much mass could provide a lot of delta-V!"
Then I began thinking on how this could be done from engineering point of view and besides some totally impractical ideas, came up blank. But c'mon, there are people smarter than me and surely I'm not the first who thought of this.
So - were there any projects (realized or not) to gain extra boost from the process of separation - by ejecting the dropped stage as a reaction mass, with a considerable force?