From the
https://kindernay.com/faq/
Upshift: It is possible to upshift (to a higher/heavier gear) under close to full pedal power.
Downshifts: It is possible to downshift (to a lower/easier gear) under load, but the rider must slightly reduce the load on the pedals during the shift.
That being said, I would not recommend shifting under load - whether it is Rohloff, Kindernay or even a regular chain cassette. In my experience it grinds down the teeth ever so slowly, and those add up over time.
I would recommend easing off pedals for a split second before shifting. It becomes very intuitive after the first couple of time you do it.