How does the tether spout work?
So its a bit tricky to explain. Best analogy is like a button on a jacket/jeans. The retainer is the button, the spout is the string, and your slit is the hole you put the button through. Now how you do that is you put the spout through first, the retainer in after, and then pull the spout through the retainer so that it can't further be pulled through your slit.
How does it feel?
Overall it feels fine. There is some minor irritation getting it in (pain, if you mess up

Does it work with the Holy Trainer (or other plastic device)
It works extremely well. I have problems of my tip rolling back inside the tube when sitting and the HT is always bad for pulling out and this was a decent fix for both. It also made peeing way more easy (even better than peeing normally imo). Only caution is that when the end of the spout rubs against anything there is some minor pressure on the inside of your urethra. From my experience so far, no damage has come from this, but it has made it a bit tender while I'm getting used to it.
Feel free to ask any other questions down below.