A Jhana is a flow state in meditation. A normal flow state in life occurs when you are completely absorbed in some activity. A flow state in meditation is analogous – it is a stable, unusually focused altered state of mediation.
What it feels like to me is somewhat like sitting underwater on the bottom of a swimming pool. The world is quieter, slower, “out there” and “ignorable”. It is generally very peaceful state. It’s easy to stay in a Jhana for long periods of time
There are several jhana states. They are characterized by depth of absorption and by what characteristics are present in the jhana – piti(energy); pleasure; happiness, etc. The book talks about this extensively in Appendix D. The standard reference for Jhanas is Leigh Brasington’s book “RIght Concentration: A Practical Guide to the Jhanas”. He also has a website and extensive writings.
That being said, my recommendation for those trying to achieve a jhana state is neither TMI nor Brasington, but rather Michael Taft’s video, “How to Jhana”. https://www.youtube.com/watch?v=0K5ypXyF3dY
To get into Jhana, you are setting up a kind of feedback loop: you wait for a pleasurable sensation to arise, and then switch the meditation object from the breath to the pleasurable sensation, which causes more pleasure to arise, and then BAM!
Anecdotally, for first Jhana you can get an experience like being electrocuted, or of having a million orgasms, or for most people,something in the middle. I got the electrocuted version 🙁
The value of a jhana state in the context of TMI is the unusual calm and stability of the state. Whatever Stage you are in, a jhana temporarily gets you to a more peaceful state. Your brain can then sort of get familiar with the direction it should be going in. Culadasa ties particular jhana stages to later TMI stages; he recommends using them to “speed your progress”. I would comment that although that is probably true, it’s not necessary, and practitioners often make progress without using the Jhanas.The connection to TMI might be stronger than that; Stages 8, 9, and 10 are sort of non-flow state rough analogs of Jhanas 2,3 and 4.
The jhanas are probably helpful but not necessary for TMI, and are interesting and useful in their own right.