I accidentally discovered that the key combination  will concatenate the current line and the next line. However, I do not recall it being mentioned in the quickref. After a quick Google, I did not find anything. Do any have a clue about where in thedocuments can I find more about this key combination?BTW, I have confirmed that this is not mapped in the :map.Update:With the help from a comment, (Curiously, I saw it in a message. But after I opened the website, it disappeared.) I found that I am actually using the <shift-j>, i.e. <J> key to join lines. Thanks to this post.Update 2: J in visual mode provides extra capabilities. See :help v_J. (Thanks to @Antony's comment.)
What does  do in Vim
key bindings
I am guessing you were holding the Shift key even while pressing j. Because V + j alone would not have concatenated the lines. V is for selecting the entire line.j moves the selection down. So, this is not the combination that did that for you.V + J on the other hand does exactly that. (holding shift for v and j)J here joins the current line to next line (J for Join), which is a default setting in Vim. You will get the same functionality without having to press V. Press J in normal mode and it will Join the next line with the current line. For more info, try help :join in Vim.