If a spacecraft enters flat enough into the Earth's (or another planet's) atmosphere it will "bounce off" the Earth at a higher speed. That method can be used if a spacecraft has not enough fuel to reach orbital or escape velocity by itself. Has any (manned or unmanned) spacecraft ever used that technique? If so, which one on what planet or moon?
Perhaps I should tell that of course you need to have the engines on. If you enter an atmosphere by gliding it will bounce you off but you won't reach a higher speed, don't you?