An Over/under launcher is one where the barrel is turned 180* (using 2 90° elbows, typically) so that it points in the opposite direction of the chamber, and is directly above it. This makes the total launcher length shorter, although the 90° bends will slightly reduce flow. The design is very common in pneumatic launchers, since many valves have the input and output ports at right angles. Pneumatic chambers are also usually longer than combustion cannon chambers, so more overall length is saved.
Recoil puts a lot of stress on the elbows, barrel supports clamping the chamber and barrel together are recommended to reinforce the launcher and keep the barrel steady.