First thing to note is that speaking in tongues is not limited to Christianity. It’s an ecstatic phenomenon that is common to all cultures and religions.
Not every experience is the same but there are common features.
Physically people may fall into a sort of trance state, or an ecstatic state where they become unaware of what is going on around them. Some people fall on the ground and thrash about, or begin to move in a jerky ecstatic dance while standing up. Other times people maintain awareness of what is happening about them.
Eyes may remain open or closed.
Like most ecstatic states they can occur either spontaneously, induced suddenly by a particular experience or cue, or be worked into via a process of prayer, meditation or “seeding”, they can also occur by being triggered by similar states in the people around them, or via the power of suggestion.
The experience can feel terrifying, or extremely pleasurable and fulfilling, a huge release of powerful emotion. Perhaps akin to the swelling overpowering feeling one gets at a concert or exciting sports match. Or it can feel very sublime and emotionally moving in a quiet yet powerful way.
People can have this sort of ecstatic experience in their own language or in an unknown or gibberish language.
People may or may not be aware of what they are saying, and may only experience it as if overtaken by a spirit, others around them may tell them later what they said.
Sometimes the person does know what they are saying, but it babbles from them spontaneously and as if coming from another source, they do not plan what they say.
Others understand the gist of the message, praise, thanksgiving, etc, but not a word for word translation.
How often if happens varies greatly. Some people have the experience only once in their life. Others on a regular basis. As some people can learn how to get into a tongue speaking state, they can call it up whenever they choose. Others only respond to an outside stimulus or suggestion. Some people have it occur when overcome by a particularly moving prayer state or awe.
I have had it occur spontaneously as the result of a moment of awe, or during comtemplation. I have remained aware of my surroundings, it has happened while walking and I continue to walk. I understand the gist of what I am saying, and occasionally quite specific things, but they are in another “language” though I think it is only an ecstatic language and not something another person could translate.
I feel deeply moved and humbled by the experience. Sometimes I cry tears of great emotion. Afterwards I feel a bit light headed and tingly, and seek a quiet time to absorb what has taken place. I have never had this experience while around other people.