Close to A-less?

That's what I found a-less is to me. Your description sounds very familiar to my own experiences, though I don't have to be erect nor have an erection. However, from what I read around, there is aparently much, much more to a-less and what you and I experiance are the very begininngs of it.

I can only achieve pleasurable feelings, such as you say; but adventually, with more practice and understanding you can achieve orgasms just as you do with aneros. Though, I personaly don't know how yet. Perhaps I'll start experimenting more with a-less. Hell, I can do it anywhere, so I might as well.

Is this some kind of A-less? Or something prior to achieving one?

From the Aneros WIKI 'Glossary' -> "Aless (a.k.a. 'Anerosless', 'A-less' or simply '-less') is any session where you have any type of prostate orgasm(s) without an Aneros massager inserted. (also see earlier identified phenomenon known as chairgasm)" This type of session is normally considered intentional in nature and is not the same as "Echo Effect™(Jack Johnston) or 'after effect', Spontaneous body sensation(s) felt after an Aneros session, these may include chair orgasms, mini-o's, butt buzz or other minor sensations."

Depending on your interpretation of What's an Orgasm?, what you've described could be classified as an Aless session. The term is deliberately vague to encompass a wide range of situations and effects. Try to avoid getting too analytical identifying and classifying the sensations, feelings and experiences you encounter on this new path of learning, doing so will allow you to learn faster by avoiding complicating mind noise.
Good Vibes to You !

When I'm sitting at my desk, if I contract the sphincter or even concentrate on the feeling of the chair against my perineum, I get a nice sensation. I get it really quickly, and usually get hard with it. But nothing beyond that, yet.
Am I in the right way? I mean, I've done it today a couple of times, and I have to say that I didn't try to go further because of studying and not wanting to do it at that time. Maybe if I try harder I'll get more pleasure. Who knows.

Hey, @bobi1990, this journey can and should be everything but "trying harder".

Instead lettin' go, waiting, watching without wanting is recommended to achieve the unbelievable and unpredictable bliss you still have no imagination of. That's why too much "mind noise" (like all the expectations you may have) is it what, as rumel already said, is to avoid. Cheers!

Btw, if you like to disclose your native language, perhaps some members here can help you even better.