What is the benefit of dolby pro logic on a40 headphones----how come when dolby is activated sound seems more underwater whereas when dolby is off sound is more in your face

Dolby Pro Logic (and Pro Logic II) are audio postprocesses that take 2-channel (stereo) sound and "convert" it into six channels. The resulting channels are Left, Center, Right, Left Surround, and Right Surround with a Low Frequency Effects channel (LFE). This configuration is also called 5.1. In the case of headphones, there may be multiple speakers for each ear, but for all intents and purposes we can consider them as having a "Left" speaker and a "Right" speaker.
The Astro A40 headphones, has an additional postprocessing stage called "Dolby Headphone". This technology takes multichannel sound (5.1, discussed above) and virtualizes it so that the content may be reproduced over headphones. The result should sound like you are listening to 6 different speakers at once. The "underwater" sound you mention is simply the artifacts associated with the virtualization process. Some people enjoy the immersive sound Dolby Headphone and Pro Logic provides, and some people don't.