That paper plate technique sounds neat!
There are many different ways that coaches teach people how to do 3-turns - depending on the level, but also on the coach.
In this context, "check" means stop [specifically stop the turn]. A very uncommon usage of the word, though hockey players talk of checking or reversing another player's motion by bumping into them. I've always felt that English is way too large and complex a language. And yet it has become very common...
I've mentioned this before, but if checking is the main issue you have trouble with, try creating the turn by counter-rotating the upper body against the rotation direction you want the lower body to turn.
E.g., for a LFO [left forward outside] 3-turn, the upper body rotates CW, and the push the lower body gives it to do so, creates a counteracting force from the upper body on the lower body that pushes the lower body into CCW rotation.
Then, when the lower body has rotated about 180 degrees, you run out of your range of motion, and muscles and ligaments have stretched until they pull the upper and lower half of the body into stopping.
A physics way of saying this is that you have the same amount of total angular momentum (roughly zero, although skating on an edge means you have a little) before, during, and after the 3-turn.
There are several key elements:
1. Your arms and shoulders must be extended and fairly stiff. Otherwise the upper half of the body would need to rotate 180 degrees too so that the upper body has rotated a full 360 degrees relative to the lower body - and no one is that flexible (I think). A physicist would say that having your arms out gives them more "moment of inertia", so the upper body doesn't have to turn much to balance the angular momentum of turning the lower body.
2. The legs must of course do the opposite, stay close together during the turn - you may even bring the free foot adjacent to touch the skating foot ankle - so they rotate more than the upper body.
3. You must bend your knees before the whole move, straighten them as you turn, and bend them again at the end. This reduces the weight and therefore the friction at the moment of spin.
4. During the turn, you must bring your weight and the point of contact with the ice forward for forward 3-turns, and forward or back [some coaches say forwards, some say back] for backwards 3-turns. This helps you get rid of that weight too - and the blade turns easier when the contact point is near an end. (At the end of the turn your weight and point of contact come towards the center again.)
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ALTERNATIVE TECHNIQUE
Of course, it is far more intuitive and common for beginners to use other methods to create turns. You could use arm swings to transfer spin to your body, let your toe pick touch, or PUSH hard outwards against the edge as you turn (e.g., for a LFO 3-turn, you could push against the outside edge, which creates friction on that side, causing your whole body to spin CCW), to create more spin (more "angular momentum"). These are very common alternative techniques to initiate turns.
But then you have to find a way to get rid of that extra spin (angular momentum) at the end.
Those techniques makes it very easy to turn - but much harder to check, because of all that extra spin.
You could check those style of turn by pushing against the edge at the end (for a LFO 3-turn, you would be pushing outwards against the LBI edge you are on at the end), to create friction that stops the turn.
By "pushing outwards" I mean outwards from the center of the arc you are skating on. In particular, that outwards foot push causes the outside lean you have at the beginning of the LFO turn to become an inside lean.
Personally I find that checking technique very hard to use.
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It is possible to combine both techniques - most of the turn momentum is created by the upper body counter-twist, but an outwards push with the foot turns the outside lean (appropriate for an outside edge) into an inside lean (appropriate for an inside edge). But concentrating on two methods at once is hard.
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I hope that was clear.