Timing is one of the things I've found most difficult to pick up while learning basic gymnastics. This is especially true when it comes to high bar/strap bar giants and tap swings. If I just blindly practice, I tend to tap far too early. For back giants, the tap is the moment when you transition from a hollow to an arch then back to a hollow again. To facilitate switching from hollow to arch at the right time I built the device you see in the video below. I suspect it would also work well for kip timing (it would tell you when to bring toes to the bar). It's basically a beam break sensor, like you might have at the bottom of your garage door, except it runs on batteries and beeps whenever something crosses between the flashlight and sensor. The idea is that you place the flashlight and sensor on either side of the high-bar at the location where you should ideally tap and then as you swing you just wait to hear the beep before switching from hollow to arch.
Here you can see and hear the tap trainer in action- I know my form is terrible, the video is just to show you how the device works. That high pitched beep you hear every time I approach maybe 30 degrees away from vertical is the sensor detecting me crossing its path and then beeping.