Basically all you do is place an item/shoot an arrow just barely inbetween the pressure plate and the pistone so once it activates the pressure plate the piston lifts the item/arrow. Then it is no logner powered and goes back down which allows it to activate AGAIN and so on. It used to work with boats back in beta but then it broke boats. I have had this design in my head for many years but am just now posting it. I'm not sure if one of these is already on here so it might not be completely original but by my thought it is. 

It's so simple but it only works for so long due to items/arrows despawning but it works for a while and it's not meant to be taken seriously. It is just a little novelty type of thing.

With arrows sometimes the arrow gets stuck in the air and it is a bit buggy. You may have to try a few times or more to get it working but other than that I don't think it will ever be taken out due to updates so it is probably continuous. Thanks for checking this out!
-Naut