By default each character that you type is inserted into the text, with the existing characters being shifted as appropriate. In Overwrite mode, each character that you type deletes an existing character in the text.
When in Overwrite mode, a character can be inserted without deleting an existing character by preceding it with
Switches Overwrite mode on if it is currently off, and off if it is currently on.
With a positive prefix argument, Overwrite mode is turned on. With a zero or negative prefix argument it is turned off. Using prefix arguments with
Overwrite Mode disregards the current state of the mode.
Key sequence: key
If the current point is in the middle of a line, the next character (that is, the character that is highlighted by the cursor) is replaced with the last character typed. If the current point is at the end of a line, the new character is inserted without removing any other character.
A prefix argument causes the new character to overwrite the relevant number of characters.
This is the command that is invoked when each character is typed in overwrite mode. There is no need for users to invoke this command explicitly.
Overwrite Delete Previous Character
Key sequence: None
Replaces the previous character with space, except that tabs and newlines are deleted.
LispWorks Editor User Guide (Windows version) - 9 Dec 2014