In learning Iaido kata or waza, one learns through various senses. On the surface, it would appear to be basically trying to replicate what the Sensei or Sempai has shown. However, for an iaidoka to really learn, it requires an understanding of each motions' intent. This requires being in the present and tuning in with one's senses to absorb what is being shared by the Senseis/ Sempais.
When first watching a new waza, one would not be able to fully understand the context or meaning of the waza. Full understanding (if there is ever such a thing) requires constant practice to uncover its true insights and significance.
The student may try to understand each motion through slow, deliberate practice, and try to process understanding through these practices. Sometimes, Senseis and Sempais may intervene to explain in order to fast-track the understanding process. This means the student will basically react and respond to the present and required depth-of-practice, rather than the superficial.
What does this mean to every Iaidoka, Juniors, Seniors alike? Everyone really needs to listen and observe what the Sempais and Senseis are teaching or showing. This is all the more essential in paired kata (e.g. Taichi-Uchi no Kurai), where rather than trying to act on one's own, iaidokas depend on trusting the other party and react accordingly in these practices.
Upon learning new wazas, there is the risk of Scientia infla - thinking that the end of the road is determined by the length of calligraphy on the wall. In reality, understanding and mastery is a fluid, continual process.
So when a Sensei is going through a kata taught earlier (individual or group practices), the Sensei has either observed or is trying to
show certain areas of the Iaidoka that still need to be improved;
teach the hidden layers within the waza;
so as to help the student reach a higher degree of mastery.
On the surface, when teaching, the Sensei is observing many things - from the student's willingness to learn, to the level of understanding and lastly, the degree of technical ability. If the student displays an unwillingness to learn, the teaching naturally stops - there is no point in teaching further, since the student is standing in his/her own path to learning and not respecting everyone's time. At this point, the Sensei may remind the student to pay attention, or simply inform the student to stop practice and leave the practice (since it is expected that the student ought to be paying attention anyway).
As any Iaidoka may attest, a kata may be perceived as a mirror, reflecting the practitioner's character. Waza practice is an opportunity for one to observe and face one's own weaknesses and strive to overcome these. Ignoring these opportunities would be regrettable. Similarly, each kata's depth is as deep as one would like to plumb. In this regard, during teaching sessions, the Sensei is also assessing the Iaidoka's character towards learning and self-mastery.
John Wheeler once observed that "as our island of knowledge grows, so does the shore of our ignorance". It takes a special kind of humility to grasp that one knows less, even as one know and grasp more. Perhaps this could be thought of as a mindset - "humility engenders learning as it leaves one open for truths to reveal themselves".