As an addendum to David Vogt's excellent answer, there are a few German verbs where the Erikative and the Imperative singular are different, an example of such a verb is nehmen "to take".
A quick Google search for the two phrases indenarmnehm and indenarmnimm shows a clear win for the Erikative (581:20).
My prosaic approach to interpreting the
form, introduced in D. Vogt's answer as correctly as jocularly as Erikative, would be:
This is actually simply the root of the verb.
Indeed, as David Vogt writes, this is not the imperative form.
I am writing separately to address the question of why that particular form is used. I am not quite sure I follow David's suggestion here that the reason may be to "try to parallel the difference between to plus infinitive and a bare infinitive" because - at least for the most part (see below) - ...
Knutsch mich ab!
The form looks like an infinitive with the ending -en removed and has been given the jocular name Erikativ after the woman who translated Disney comics into German, Erika Fuchs, and the not so jocular name Inflektiv by Oliver Teuber in a 1998 paper that is unfortunately not available online.
One difficulty ...
Seit zwei Jahren tausche ich meine alten aber noch guten Sachen hier und bin begeistert.
Mein is a Possessivartikel and in this case it's meine, since it's
That's because it refers to Sachen and they are Plural.
(Note that it would also be
Seit zwei Jahren tausche ich meine Sache hier und bin begeistert.
Because Sache is feminine. ...
The sentences you wrote are unusual in that the more specific (e.g. in den Keller), precedes the more general (e.g. runter). The reverse is more natural and I think occurs more frequently:
Er geht runter in den Keller.
Geht doch mal raus an die frische Luft!
Wir wollen rauf auf den Gipfel.
Kommen Sie doch auch rein ins Wasser!
For this reason, ...
You will sound like a news anchor then.
Those tiny bits are not really neccessary to understand a clearly spoken sentence. However, as you may have found out already, German speakers mumble all the time and then, those words are a life saver. Consider someone telling you
Gehns heut nimmer ofn Berch!
because it's too late already to climb the mountain. ...