WMI is the injection of a water-methanol mix (typically 50:50) into the compressed air after the turbos to reduce intake air temperatures (IATs), add octane, add fuel, and clean the intake valves. The intake temperature reduction and added octane allows more boost to be run with more advanced ignition timing to make more power. The ability to add fueling helps overcome fuel system limitations (injectors and HPFPs on the N63TU), allowing more power to be made. WMI is especially useful on the N63TU because the stock charge cooling system (intercoolers, pump, and radiator) is insufficient to keep intake air temperatures down at even mild boost increases above stock levels. For example, at 22 psi of boost, intake air temperatures will typically rise by 60 degF over just a 60-130 mph pull. Over a longer pull, like 1/4 mile, if ambient temperatures are warm, IATs at the end of the 1/4 mile can be 170 degF or higher, causing significant loss of power due to 1) less dense air, and 2) more importantly, ignition timing cuts due to the increased chance of knock from the high temperatures in the combustion chamber.
There are a few downsides to WMI. One is having to source methanol (most drag strips and some speed shops sell it, typically for $3-$4 a gallon), mix it with distilled water, and fill the WMI tank. A 1-2 gallon WMI tank will typically last as long as a tank or two of gas during normal driving, but for days at the track or multiple runs in Mexico, it might have to be filled a few times for a single tank of gas.
WMI is typically injected through nozzles installed in bungs in the charge pipes before the intercoolers, or in bungs welded into the bottom of the intercoolers on the N63TU. Injecting at the bottom of the intercoolers allow the intercoolers to do their job a bit more efficiently, but injecting in the charge pipes provides for much simpler installation and maintenance (changing and/or cleaning nozzles).
Adding WMI typically requires tuning changes to take full advantage of it, and with tuning changes, gains of 30-70 crank HP can be expected on N63TU vehicles, depending on vehicle modifications, fueling, and ambient conditions.