Unmesa and Nimesa have been described by Jaideva Singh in his commentary to Spandakarika 1.1 as appearance and disappearance. These words don't totally convey the meaning though as it is not mere appearance and disappearance. They relate on the deepest level to the upward and downward "movement" of consciousness. The ascent and the descent. In relation to the central channels Arohan and Avarohan. The two processes are simultaneously related as Consciousness is one.
In the process of mudra, there is simultaneous contraction and expansion, narrowing and opening. In terms of the yoga, the bandha process ultimately points to and derives from the narrowing confining aspect. The fullness and opening of this process leads to profound opening. Mudra contains within it bandha and is thus a more complete concept but nevertheless it is important to see the "parts" within it.
This is why one "sees/feels" on one level the centralization through susumna nadi and the profound flowering or opening simultaneously.
This is all on the level of what yoga terms sahita, which is a term similar to the Kashmiri modes of conscious practice as opposed to kevala, which is more in line with completely unfabricated "modes". The words get difficult at this point and I will attempt to further clarify more later.
The main point in this discussion is that there is a certain "simultaneity" of expansion and contraction, containment and release, that is a part of this level (the sahita) level of mudra. If this practice is not understood in the practices of pranayama, the practices will only really be felt on the grossest of levels. True pranayama and further the nirodha of the mind with all of its stages will only reach fruition with this depth of work. Further, mantra sadhana will be far accelerated with this type of practice, as one learns to integrate mantra and mudra. It is here that one really sees the depth of mantra and what it is capable of.
On the level of the tantra, the yantra practices help one to internalize this process more readily. The Sri Cakra is the ultimate representative of these twin principles.