Pressing C+b searches the current
buffer and the current mode's keyword list for words beginning with
the one before the caret. You can use this feature to quickly insert
variable and function names without having to retype them.
The fact that it completes keywords also
comes in handy in many situations; for example, say you are editing a
JavaDoc comment and can't remember if it's @return or
@returns. No problem; just type @re and press
C+b to find out.