Direct from the website of Devil's Own :
6. Intake Manifold
This location can be the most complex area to install. Usually requiring that the intake be removed for access. Injecting here is going to yield the largest water/methanol droplets. And with the closer proximity to the combustion chambers is going to provide a larger amount of mixture into the cylinders. Doing this usually requires more fuel from the factory system be removed. This option is best for max cooling.
The water here is being injected in a manner much like port fuel injection and the bigger water molecules being injected have a more direct effect on in cylinder cooling, and altering the flame front of the combustion charge in a way much like a higher octane fuel. To take full advantage of this nozzle location it is suggested that trimming away some of the factory dumped fuel be done. In other words, in order to take advantage of the better properties of water over fuel for cooling and injecting water at this location, you want to remove the dumped fuel and actually REPLACE this with water. This injection point with a pretty sophisticated WI control method allows for the most advantageous use of water injection. The drawback for nozzles located heres is kit/component complexity, install complexity, additional labor, possible machine shop costs, and possible additional parts costs.