O.K. @fermunder...
You've gotten my 2c worth regarding flippers.
Now, here's my opinion about PK rings.
Basically, they are all very similar. Totally magnetic and somewhat fragile...don't drop them ;-)
However, today's rings are much more durable but will still shatter on hard impact. Don't worry about this too much...shattering is rare, chipping is more prevalent (unless you're a klutz...well?)
Just teasing...
Whether it be Black Magic's PK ring ($30.00)
http://www.penguinmagic.com/p/1435
or Kinetics PK ring ($39.95)
http://www.penguinmagic.com/p/S19119
Both are very good rings.
However, it appears that PM is "pushing" the Kinetics PK because the other ring, Black Magic, is always "out of stock" (been that way for a while).
Who knows?
After all this gibberish, I personally use the Neomagnetic Ring by Leo Smetsers (approx $53.00)
http://www.penguinmagic.com/p/S18846
Why?
Simply, the best thing about the Neo Ring is that I'm in complete control of activating and deactivating the magnetic field i.e. turning it "on or off".
Plus, since it's made of stainless steel, it's not brittle or subject to destruction as the others may be.
But, wait...you're thinking to yourself "stainless steel isn't magnetic".
Well, not totally true, but since that's the common thought of most people, let's go with it. I'm not teaching a science lesson!
This may give you a hint as to the ring construction, but since this is a public forum, I cannot say anymore without giving away the secret of turning the mag field "on or off"
Candidly, think about it. As with a lot of magic tricks, gimmicks, utilities, etc., if you think long and hard about it, the technique reveals itself.
Heck, it's not rocket science!
Best wishes...
