To make sure we're on the same page, terminology wise:
- Factors - these are the factors or qualities of the various Samatha Jhanas - joy, well-being, equanimity etc.
- Momentary Concentration (Kanika) - ability to stay focused on moment-by-moment arising and passing of phenomena.
- Access Concentration (Upacara) - ability to stay focused on on a somewhat contrived stable object i.e. the breath, color, shape etc.
- Absorption (Apana) - stable, one pointed mental state.
The Jhanas themselves are a continuum of mental states that don't exist as separate discrete entities. They are mental states that unfold depending on where in the continuum your experience lies. Somewhat like a rainbow - it is made up of different colors that have different qualities but on close examination they blend together seamlessly. The mind moves through these qualities in Jhana by focusing on the different qualities.
Generally speaking, deepening of access concentration increases the chances of the unfolding of Jhana. Once the first Jhana is established no special factors of access concentration are required to move into the second. The qualities of the first Jhana are :- pleasant sensations, joy, and sense of well-being. The meditation object of concentration (Samatha) is left behind and the minds merely abides (is absorbed in the Jhana qualities). When shifting through the Jhanas the focus just changes to subtler and subtler factors - from sensations to joy, from joy to well-being, from well-being to equanimity. So IMHO there is no dropping out of Jhana to gain access to next etc. or special factors of access concentration required. Five factors help to navigate the jhanas:
- Directed attention (Vitakka) - directing the attention to a particular perception.
- Sustained attention (Vicara) - sustain attention on an object.
- Rapturous interest (Pita) - When vitakka and vicara are steady, a feeling of lightness and pleasure naturally occur.
- Deep ease (Sukha) - Sukha is a quality of happiness that is much quieter and smoother than piti.
- One-pointed attention (Ekaggata) - Ekaggata’s characteristic quality is to lock on to the chosen object with an intimacy that rivets the attention, stills the mind, and settles into unwavering focus.
These five elements of concentration — connecting, sustaining, rapture, joy, one-pointedness — take center stage during jhanic experience. Each level of absorption has its characteristic blend of these factors. Skill in entering and maneuvering through the levels of absorption depends upon cultivating these five factors. Catherine, Shaila. Focused and Fearless: A Meditator's Guide to States of Deep Joy, Calm, and Clarity (p. 108). Wisdom Publications.
Momentary concentration is cultivated specifically for vipassana practice but can result in Jhana as well. Daniel Ingram:
The vipassana jhanas differ from the concentration jhanas (samatha jhanas) in that they include the perception of the Three Characteristics, rather than the “pure” samatha jhanas that require ignoring the Three Characteristics to get them to appear stable and clean. However, the two may share many qualities, including very similar widths of attention and other aspects.