Your right about the pulse increase in the beginning, however i am more interested in the post- shower pulse. Icing the neck might help, but it doesn't jolt the nervous system like I explained.
Now, i have no research to back this up, but I do find it interesting. There are many systems in the body which are known for resetting itself given a certain set of conditions. Almost like an extreme situation can give you a diffrent outlook on life or "wake you up". In theory this sounds resonable, however as stated before I haven't done any practical testing.

Another thing which is entirely unrelated to showers I found today while reseaching the brain. People who had experienced a concussion with a fractured skull or bone are much less likely to suffer from Pcs than those who hadn't any signs of bone fractures. The explanation is quite simple: those who have an actuall broken skull are much more likely to take the illness seriously and rest in the following weeks. So if there are anyone out there who have recently had a concussion, please rest!