A numeric arg says to kill the characters instead of deleting them. Add the next character that you type to the line verbatim. This is how to insert things like C-q for example. Insert a tab character. Drag the character before point forward over the character at point.
Point moves forward as well.
If point is at the end of the line, then transpose the two characters before point. Negative args don't work. Drag the word behind the cursor past the word in front of the cursor moving the cursor over that word as well. With a negative argument, do the previous word, but do not move point. With a negative argument, do the previous word, but do not move point.