Interesting question. Reminds me at the outset, of one of those annoying and truly sad little incidents that happen in life. This was (of course) many years ago, way before video projectors, so it was a 16 mm movie on "human sexual response" that was being shown at "lunchtime" and let me tell you, to a packed
lecture theatre. It was dealing with just the sort of thing you describe, when 1/3 of the way through, I was paged by a very demanding ward sister to attend with "urgency" to some paperwork.
Dutiful little intern that I was, I left the theatre and sorted out the matter - it was
in the event worthless paperwork, but took a good half hour - way longer than the movie. At the very worst, I really should have just looked at it and gone straight back to the move - I learned much that day about priorities and people, but regrettably, missed most of the important stuff.
Frankly, I cannot see how the presence or otherwise, of a foreskin would affect the cremasteric response which is by the way in respect of your question, hardly under voluntary
"control". Jim has mentioned the aspect of temperature - the testicles retract in response to cold, in order to regulate their temperature fairly
closely. They also
retract as part of the sympathetic nervous system "fight or flight" response to apparent danger or stress - for obvious reason.
And it is said that they also retract toward orgasm, and an urban myth states that ejaculation can be inhibited by holding them down and preventing this retraction. That latter is sheer bunkum.
react to to sexual interest - that appeared to be the gist of that much of the documentary that I did see, it certainly noted that they periodically
or rhythmically move up and down - perhaps every minute or so, and just as the bowel and other smooth muscle in the body does.
And of course Jim is fairly
correct in mentioning that you might expect any sexual interest (not requiring nakedness on the part of the object of interest) to be reflected in some degree
of erection, but not necessarily complete by any means.
In short, the rhythmic movement of the testicles is entirely normal - you could quite likely observe yourself and find the same thing. It certainly is
influenced by activity, and certainly by noticing something of interest. And I can see no reason that it would be prevented by circumcision.
What I will
mention, is that circumcision removes a good half of the penile skin in most cases, perhaps more. Particularly during erection, this is accommodated by part of the scrotum being drawn up onto the penile shaft - to a greater or lesser degree. To this extent, circumcised men would tend
to have less mobility of the scrotum on average - but note I say, "average" - there are always (plenty of) exceptions.
Does this answer help you?