What should RESETING detached HEAD to BRANCH do?

If I understand right, when you have a detached head(HEAD->commit), then the git CHECKOUT branch solves the issue(HEAD->branch[->someCommit])

But what does resetting to a branch do? It was supposed to set the pointer of an object(often a branch) to which HEAD points to another branch. But since we dont have this middle-man (branch) - what does it do then? And why?

Solution

It simply moves HEAD: as I mentioned in "Practical uses of git reset --soft?":

git reset is all about moving HEAD.

If you move HEAD to another commit, then HEAD remains detached.

After a discussion about the difference between git reset and git checkout vs the detached or attached nature of the symbolic reference HEAD, here what we found together:

if HEAD points on branch, git reset would move branch as well to <something>: resetting it doesn't make it un-detached, it changes branch HEAD
if HEAD points on a commit (was already detached), then it would remain detached: As the OP puts it:

so when detached and I call git reset <branch> it finds the commit behind the branch and change the commit in .git/HEAD for the commit that <branch> refers to.

Let's consider a HEAD which is attached to branch1 (cat .git/HEAD would return branch1):

git checkout branch2 will change HEAD to branch2 and leave branch1 untouched
git reset branch2 will reset branch1 HEAD to branch2: cat .git/refs/heads/branch1 would contains the same SHA1 as branch2.

That's the difference:

reset moves HEAD (as I told at the beginning: it is all about moving HEAD)
checkout switches branches (or detaches HEAD)

Regarding the attached/detached nature of HEAD:

reset doesn't change the nature of HEAD (if it was attached, it remains attached)
checkout can change the nature of HEAD (if it was attached and you checkout a commit instead of a branch, HEAD becomes detached)