At one dojo, we had a version of this that involved stopping our movements when sensei said, and hold ourb position (of course, if falling, hitting the ground was permitted
) as a way to evaluate (self- and sensei-) your balance as uke...
Another variation was to do this with a partner...and he'd let us stand there for some time looking for any signs of waivering, pointing out a nage or uke (or both
) who lacking balance was leaning on a partner...