Daniel M. Ingram:

Sub-jhanas are a way of breaking down jhanas into their component parts, a further refinement of the maps of insight (vipassana jhanas) and concentration (samatha jhanas).

The general concept may be illustrated thusly:

When in the first samatha jhana, initially the jhana is fresh, requires some attention to stabilize it, and is not that well developed. This is the first sub-jhana aspect of the first jhana, here called 1.1. When it becomes stronger and comes into its own, this is the second sub-jhana aspect, her called 1.2. When the flaws in the first jhana are noticed, namely that it requires a degree of effort that muddies the more enjoyable aspects, this is the third aspect of the jhana, here called 1.3. When there is a balanced acceptance of the jhana that begins to lean towards what comes next and the shift to something higher, this is the fourth aspect, here 1.4. In this way, aspects of the jhana that mirror the larger 4 jhanas are seen.

However, those who have stronger concentration may see higher aspects corresponding to the formless realms, and in that way, may get the first jhana so strong that the 5th jhana boundless space aspect can show itself, being now 1.5, with the body largely vanishing, and so forth, up to 1.8, which is the natural height of development of the first jhana.

I remember I was on an insight retreat and practicing very hard, really trying to work on staying with the breath. I was putting forth a lot of effort, and entered the first jhana, which in this case was largely a samatha jhana, as I was not perceiving the Three Characeristics. I got my concentration on the breath so strong that eventually everything vanished except the breath, body gone, room gone, sounds gone, just breath as a vague, formless motion in space, and then even this was nearly gone, but still strong effort, still first jhana, just the high formless aspects of 1.5 to 1.7 or maybe 1.8. Soon thereafter I got into the lower insight stages of Mind and Body, etc. In this way, we see how the sub-jhanas can work.

One may also speak of the sub-jhana aspect of the insight stages. For instance, the 11th stage, that of Equanimity, has many obvious sub-aspects to it that can easily mirror the first 4 to 8 jhanas, and thus people may find themselves in the Nothingness aspect of High Equanimity, which would be called 11.7, or even the Neither-Perception-Nor-Yet-Non-Perception aspect, 11.8, all as a normal part of insight practice. These are actually common aspects for people to get into who have strong concentration.

One can even differentiate further, getting into sub-sub-jhanas or sub-ñanas. As to sub-sub-jhanas, and by way of example,