The manual is clear about it: "CTRL/K causes the cursor to move non-destructively to the same character position in the line immediately above the current line."
Not into manuals? No problem.
ADM-3A, one of the 1970s' first widespread terminals for computers, came with "a keyboard that resembles the keyboard of a conventional typewriter". Above the H, J, K and L keys hovered additional symbols: ←↓↑→.
Bill Joy developed the text editor vi on an ADM-3A, dropped the CTRLkey and used HJKL to navigate text. That's why Gmail today uses J and K to navigate messages.
Whether you'd like to drop that legacy or add more ADM-3A mappings(- is RUB, for example), you can:

›› Want to command Gmail with your keyboard, and rule it fully? Here's how to set up your own Gmail keyboard shortcuts.

Lin Yutang gives an example: "if one answers letters promptly, the result is about as good or as bad as if he had never answered them at all."
This is one way to counter pointless efficiency, for Lin a vice that keeps Americans treading, nervous and unhappy; "besides the noble art of getting things done, there is," after all, "a nobler art of leaving things undone."
Now, about those letters you've just sent: you can still undo their sending and improve your results.
›› Wish you had not sent what you just sent? With the right settings, Gmail has not yet sent it, and you can still take it back. Yippee!

Saint Patrick is not green, of course, with envy.
He likes his newfangled Irish green soutane just as much as the traditional blue (Saint Patrick's blue, in fact). See for yourself:
›› For St. Patrick's Day, put St. Patrick himself in your emails. (Windows Mail, Windows Live Mail, Outlook, Outlook Express)

Enter
Enter
Enter!
iCal is not too impressed by my attempts to, well, enter a line break — until I stumble upon the Option key and the Enter key, all at the same time.
Now, adding line breaks to events in iCal using Option-Enter does not work with Google Calendar items — but synchronization does:
›› Want to use the speed and ease of iCal with the ubiquity and versatility of Google Calendar? Here's how to add Google Calendar agendas to iCal and have them sync automatically, too.