An undersaddle piezzo pick-uo coupled to an active pre-amp with controls for volume and additionally, reverb, chorus and delay. (all adjustable). When plugged in, sound control can be managed directly on the uke rather than the amp. But the great feature of this system is that you do not need an extrnal amp at all to hear the effects. When activated, the system uses the body of the uke like a speaker though for maximum effect the uke has to be held slightly away from the player's body.
Volume cannot exceed the level of the uke without a pick-up but does amplify lightly played strings. Reverb and chorus are heard at any volume level but on my installation nearly full volume is needed to hear the delay effect. This could be an issue of transmitter positioning since it has to be stuck to the inside of the ukulele back using its self adhesive pad. I am not a luthier but fitting is straightforward - a 26mm x 24.5mm cut out for the controller, then 12mm and 3.5 mm holes for the end-pin and pick-up respecively. Control kknobs are tiny but make for a very neat installation.The syztem is powered by a rechargeable (non-replaceable) lithium battery - said to last for ten hours. The usb charging lead plugs into the endpin jack socket.
I have ukes fitted with more expensive rechargeable systems, but unless the novelty of adjustung effects directly on the uke wears off, the Flight Soundwave gives comparable performancce whilst adding an extra dimension.