Sorry. Didn't mean to drop this thread, I got really busy.
I don't understand the mechanics behind it (but others here no doubt do) but when I was immersing my elm to water it, I got some serious salt accumulations on the moss and soil surface. White chunky hard areas. That's a hint that the water is not flushing the soil, taking away the salts and carbon dioxide and bringing in freshness.
The draining off of the water from immersion doesn't happen the same way as it would if you watered from the top. Maybe watering from the top helps draw the water through because the soil on the bottom is drier, so it draws the water by capillary action? And if it immersed, the water drains but doesn't "draw"?
If it works for you, you can go ahead and keep doing it. However, sometimes there is a way that works better. You'll never know until you try. You can keep a tree alive and reasonably healthy in a wide range of situations, but there are some that are more effective than others. Just one of those things.
And thank you for considering how many posts I have, but truthfully, I am only an intermediate bonsai artist, if that. I am a prolific reader and writer, however.
My job as moderator is to help people get answers from the people who know, and to try and keep things moving in a positive way. The advice about watering is given here by people much more experienced, I am simply repeating it. And because it is backed up by my own experiences, it seems worth repeating.
Good luck!
Joanie