Lots of people have been sharing first-hand reports of their personal experience of the so-called "Kundalini awakening". The following are a few illustrative anecdotes: 1, 2, 3, 4, 5, 6, 7, taken from YouTube and the subreddit on Kundalini.
Marjorie Woollacott, PhD and professor of Neuroscience and Human Physiology at the University of Oregon, has devoted part of her time to the research of the "Kundalini awakening" phenomenon. In her recent paper titled Investigation of the phenomenology, physiology and impact of spiritually transformative experiences - kundalini awakening (https://pubmed.ncbi.nlm.nih.gov/32753262/), she makes the following claim:
The rationale for this study was that though there are many scientific articles published on stress reduction and increased mental function related to meditation, yoga and other tools that come from spiritual traditions, peer-reviewed research on energetic or kundalini awakenings is rare. This may be due to the phenomenology of this awakening, which does not easily fit into the current materialist scientific paradigm. Nor is it easily amenable to traditional means of scientific experimentation and measurement.
To understand why Woollacott claims this, let's look at the typical symptoms of the "Kundalini awakening" syndrome.
The most frequent and distinctive one appears to be the feeling of an "energy" or "electric current" moving up the spine toward the crown of the head. Here is a quote about this from Woollacott's paper:
Energetic awakenings are often accompanied by physiological experiences. Historical texts [6,7] and modern research  concur that as part of the awakening and transformative process, many individuals feel energy moving up the spine toward the crown of the head, or centered in the heart. This is frequently associated with inner sounds or an experience of radiant light, a sense of joy, or a sense of deep interconnectedness with others.
The Wikipedia article on Kundalini seconds this:
Other symptoms and results of the "Kundalini awakening" syndrome are summarized in the following tables and figures from the paper:
Other people holding reputable credentials who acknowledge the "Kundalini awakening" as a real phenomenon are Bonnie Greenwell (PhD) and Lawrence Edwards (PhD). Here are a couple of interviews with them: 1, 2.
Bonnie Greenwell wrote a PhD dissertation titled "KUNDALINI: A STUDY OF EASTERN AND WESTERN PERSPECTIVES AND EXPERIENCES AND THEIR IMPLICATIONS FOR TRANSPERSONAL PSYCHOTHERAPISTS" (available here). She describes the phenomenon as follows:
This dissertation explores in depth the experience of kundalini awakening, a complex spiritual, psychic and physiological phenomenon which may occur in individuals spontaneously or as a result of certain spiritual practices or therapeutic processes, and discusses theories and methods which may be useful to transpersonal psychotherapists who have clients involved in this experience. People are commonly misdiagnosed and inappropriately treated by therapists and physicians when having this experience.
The dissertation identifies seven major ways in which kundalini phenomena is manifest, usually over a period of years---samadhi experiences; pranic activity which includes a wide range of body movements, vibrations, and temperature changes; spontaneous production of yogic postures or hand movements, yogic breaths, mantras or other sounds and symbols; sudden apparent physical illnesses, pains and eating difficulties; emotional upheavals, often accompanied by fear of death or insanity; seeing visions or hearing inner voices and sounds; and acquiring of psychic abilities or other occult powers.
The study reviews classical yogic literature on kundalini, and presents facts and commentaries on 23 case histories, 14 from Eastern and Western literature and 9 obtained from in-depth interviews with Western people who appeared to experience kundalini awakening.
QUESTION: What is the explanation for the symptoms people commonly report to have during Kundalini awakening, such as the feeling of an "energy" moving up along the spine, vibrations, electric currents, heat, euphoria / bliss, among others?