How did War Emergency Power work?
Generally, War Emergency Power was a power setting that was available, but exceeded the engine's long term running condition rating.  It was usually specified in manifold pressure, and primarily applied to supercharged or turbocharged engines (which was most of the high performance piston engines by the end of the War).

The way it works was much like how it works in modern jets (where it often specifies afterburner operation): There was a stop on the throttle and a warning mark on the manifold pressure gauge that warned the pilot not to exceed those settings, as well as a specification in the manual for the longest permitted time in War Emergency Power setting.  To engage this setting, the pilot either had to manipulate the throttle lever in an unusual direction (pull up, push sideways) or break an actual wire with a push strong enough not to be done by accident, in order to bypass the stop.  In aircraft where it was used, the wire served to notify ground crews that WEP had been used.

Once engaged, War Emergency Power would give anything from 10% to 20% more total horsepower (in most cases, though the last version of the P-51 got roundly 60% increase), and a significantly increased rate of engine parts wear and risk of failure (blown gaskets, thrown connecting rods, even holed pistons from detonation if the fuel wasn't quite up to spec).  This doesn't seem like much, but 20% additional power would amount to 10 knots or more of additional level flight airspeed, or several hundred feet per minute of increase in climb rate -- it could make all the difference in air combat, when fighters of one force were often quite closely matched to those of the other in terms of performance.