picture (c) Richard Stonell
Uchikomigeiko is a drill where the motodachi shows various openings to the student or kakarite, who does their best to cut and follow through as quickly and as correctly as they can.
After a while, motodachi sometimes get lazy or forget their job, and they show the openings to the student before the student has entered ma'ai. This makes the student's job much easier and they can come into ma'ai recklessly and without stopping. This helps create the false feeling that the uchikomigeiko is flowing smoothly.
I say false because to enter ma'ai recklessly, just assuming or trusting that your opponent will show an opening is not good kendo strategy. Even when training fundamentals, we should be developing a sense of what really works.
The motodachi plays an important role here. They should always make the student seme strongly before showing them the opening. If the student ignores this, the motodachi should keep their shinai in the centre and allow the student to run onto the kensen (according to the experience of both partners, the strength of this mukaezuki should be modified, i.e. kyu grade motodachi may allow the kensen to touch their opponent's body but then should quickly remove it. Experienced motodachi can keep the kensen there and really stop their partner in their tracks, provided their partner has the experience to receive it properly). They will then get the message and be a bit more wary.
To find a balance between recklessness and waryness is what training in uchikomigeiko is all about. The student steps in and shows good seme, not knowing how their opponent might react, but ready for any opening. Being able to improve this kind of all-around readiness is the great benefit of uchikomigeiko.
Remember: to the casual observer uchikomigeiko should look like it is flowing, but to each of the two people doing it, there should be: 1) seme-> 2) opening-> 3) seeing the opportunity and taking it.