Vertical enemy movement

I’m working through the basic intro to Flowlab series and need help with something. I’m trying to make an enemy (a buzz saw) move up and down, but I can’t figure out how to set up the movement. Can anyone explain how to create this basic up-and-down motion?

Have a number attached to a velocity in the y value. To switch it either add collisions or timers that switch the number between a positive and negative.

If you want it to move up and down smoothly, you can simply use an easing value that adds to the y value and reverses.