Dunno much bout these. This one looks like it is some kind of compressed air system or something, which might be kind of neat if the system charges by way of pedal power.
This one looks like it might be some kind of fuel mixture rocket jet kind of thing. I’m thinking the two cylinders hold compresssed gas, then get mixed together just before they get to the big nozzle. Just guessing though.
If anybody knows what the real deal is, post in comments please.