I recently read a post about off-cushion work for progressing on the spritual path.

Sri Ramana Maharashi was mentioned there, so I read his short book “Who am I“ where the method of constantly asking oneself who is having thoughts once they arise is described.

I was wondering if anybody has some experience with this technique that he/she would like to share.

Gary Weber discusses this technique a lot:

http://happiness-beyond-thought.com/

Jake is a pragmatic dharma dude who is a student of Gary's last I checked.  You might try to contact him via his blog, as he has been doing off cushion inquiry for quite awhile:

https://jakeyeager.wordpress.com/

I'm a fan of the advaita/non-duality view of how things end up, but I honestly haven't seen much in the practice that would suit anyone that is pre-stream entry, or that I would recommend to my students. The inquiry about "Who Am I" is a classic in many schools, however, and is always worth exploring.

Can someone please ELI5 (explain like I am five) what kind of stuff one should be doing / experiencing / looking for after asking questions like "who am I?", "who is this happening to?", and so on?

When I do, I don't get much of a reaction from the "system". It mostly sounds just like words. There is a slight increase in the intensity of proprioception, or maybe an increase in the tactile intensity of some sensations in the chest or maybe throat region. But when I look at those sensations, it is pretty obvious that those sensations are not a self because of my theravadin practices. I do not really see the usefulness of asking this, so I must be missing some important ingredient. Or is the idea just to "kickstart" those sensations and then having a good look at them?

I think this question is a great place to start for answers on a number of topics. Trying to find your origin in a reductionist fashion demonstrates that we aren't one "thing" and that we have no "I" as a center. Examining our thoughts makes us realize our impermanence, that "I" is an evolving thing, moment by moment, never static. You can look for the sensation of I and see it isn't your body, but somewhere at the top of your skull, or slightly behind you, or realize a number of things about your perception. Check out Douglas Harding for fun with this one:

http://www.headless.org/experiments-video.htm

Try the experiments from the links on the left.

There probably plenty more, but that's just off the top of my head. 

I'd be curious to hear more about your observations and practice on this one.

As far as I understood the instructions in “Who Am I“ the question “who is having this thought“; “Who am I“ is supposed to keep the ego in the place where it originates from (the heart region) which will in the end liberates one from the ego alltogether.

I find this approach really interesting as it does not appear to be an investigative process like vipassana but rather surpressing the ego.

...I neglected to add possibly the most important thing this inquiry can reveal - that not only is there no you between your thoughts (you ARE your thoughts) but also that there is the aspect of the "watcher".

Start with who am I. Do your best to answer the question. See what you come up with. Then examine the answers one by one and see what they are made out of (sensation, thought, perception etc.). Ask who is seeing those answers and the actual things. If you hit confusion note the thing which sees mind(thoughts) as they are confused and clear. When you think you have the answer, inquiry what is that behind thought which knows the answer. 

After some time, break through the not-knowing by just letting go and being. Stay without thinking. Could this be it? Why or why not. Find the split between seer and seen. What is the border made out of? How do you know it's not interpretation.

Ask the question again, this time just stay with the question and with the feeling not-knowing the answer. See mind coming to derail and to figure it out over and over again. 

Then move on to Awareness Watching Awareness. Are you aware now? How do you know it? Is it a particular thought that tells you that, is it based on opinion whether you are aware or not? There is a sort of a nice fuzzy feeling that you get in, stay in that silence for as long as you can. Then come back again.

Experiment really, live this question. Feel it, contemplate it. See what happens.

Nice guide!.

sent you a pm, John.

I didn't receive it, make sure you go here: http://www.dharmaoverground.org/messages

Agreed.

Sent another, did you get it this time?

Your question reminded me of technique in this video

https://www.youtube.com/watch?v=GfqLdOgoB98

Im not so sure about of some of his claims but I found it interesting and I am thinking of trying it