I'm not sure about the blood-flow to the pineal gland theory. Not specifically by 'pumping' the muscle groups you are talking about. But:
"Unlike much of the rest of the mammalian brain, the pineal gland is not isolated from the body by the blood–brain barrier system; it has
profuse blood flow, second only to the kidney."
Which is news to me!
I think the sensation of pumping your neck muscles, just below the skull is actually manipulating your Eustachian tube. That is what is opening and
closing when you yawn:
There are four muscles associated with the function of the Eustachian tube:
Levator veli palatini (innervated by the vagus nerve)
Salpingopharyngeus (innervated by the vagus nerve)
Tensor tympani (innervated by the mandibular nerve of CN V)
Tensor veli palatini (innervated by the mandibular nerve of CN V)"
I find it interesting the Eustachian tube is connected to the vagus nerve - that seems to give credence to the idea of inducing euphoria and a sense
The Vagus nerve is highly associated with the production of Oxytocin - one of the main chemicals involved in love and bonding. There have been
experiments where people with severe depression have had electrode stimulation of the Vagus nerve and it has had remarkable effects.
"Recent studies have begun to investigate oxytocin's role in various behaviors, including orgasm, social recognition, pair bonding, anxiety, and
maternal behaviors. For this reason, it is sometimes referred to as the "love hormone". The inability to secrete oxytocin and feel empathy is
linked to sociopathy, psychopathy, narcissism, and general manipulativeness.[not verified in body] However, there is some evidence
that oxytocin promotes 'tribal' behaviour, combining trust and empathy with the in-group with suspicion and rejection of outsiders."