Verbal vs. Somatic learning
I agree with Ian about the level of discussion on the mat. But probably for a different reason. Research on neural plasticity (the capacity to learn from a neurological point of view) has demonstrated rather clearly that encoding information of a somatic nature (eg, learning how to take ukemi) is done in areas of the brain that are distinctly separate from those centers that encode verbal information (instruction by voice).
The thalamic-cerebellar tracts, somatosensory and motor cortices are responsible for the control and learning of body movement, from gross motion to fine control. The thalamic connections to the temporal lobes (grossly involved in verbal skills and encoding of memory) are clearly separate. Research has shown that the brain operates most efficiently when learning in one or the other modalities. That has certainly been my experience. I love to talk about it, but my ikkyo is much better when I don't.
I am not suggesting that you never speak on the mat. Of course you will. Our language is a more highly developed form of communication than is our aikido. This was not true of O'Sensei, who could convey a thousand lessons with one technique. So, at [my] humble level, verbal instruction is a must. However, practice often requires focus, timing and awareness. Those three are not amenable to concomitant verbal discussion. The budo masters knew this, and we are just beginning to understand why from a neuroscience point of view.
I realize I committed a rant just now, but this has been a perennial issue in my dojo...