Here are instruction for a fun element that I like to call the Trampoline Arm. The trampoline is from a knex set called Big Air and You'll need 1 of those. It works 99% of the time and is easy to build. You just need 1 special piece (see step 1). You can always watch the video to see it in action.
If there are any problems/ questions feel free to ask me in the comments below.
Thanks for watching/ building,