Moving the branch

git reset moves the branch you are on. What else it touches depends on the mode, and the three modes map exactly onto the three trees.

--soft moves the branch and nothing else, so everything the commits contained is still staged. --mixed, the default, also rewrites the index, turning that work into unstaged changes. --hard additionally overwrites your files, and is the only one that destroys anything.

In every case the commits you moved away from still exist and the reflog still names them. --hard is dangerous because of your uncommitted work, not because of the commits.