Here are instructions for a cool element that uses 6 knex balls. It's really easy to build and works 99% of the time. but you can't make this element shorter or longer, or you'll have to adjust the counterweight that you can find on step 3. Unfortunately this element is not included in the video, so the only way to see it in action is to build it :).
If you have any problems/ questions feel free to ask me in the comments below.
Thanks for watching/ building,