Here are instructions for one of my best elements so far. It works with gravity, a locking mechanism and uses 8 knex balls. You just need 1 special piece that I like to notice (see step 1). It works 95% of the time and is easy to build. If you want to see it in action you can always watch the video.
If there are any problems/ questions feel free to ask me in the comments below.
Thanks for watching/ building,
Step 1: Special Piece Needed
Now you need to put 7 Knex balls like this. And drop the 8th ball on top like this.