Well, basically a "umgeschulter Linkshänder" (German term) is a lefty who doesn´t write with his dominat left hand, but with his right hand.
Once upon a time someone decided that everything in life would be so much easier, if people would do everything right, aka right-handed. Today that seems about as strange, as if to decide we would have only women or men in our society. And it seems most likely that the ratio of left-handers and right-handed people should be just about 50/50, such as gender distribution (no offence intended).
In the past, however, left-handed children were often forced to use their right hand. Johanna Barbara Sattler, an expert in the field, calls it "failure to render assistance", if one does not help a child to use his dominant hand, especially for writing. (The English term is "brain breaking"!). Nowadays it is considered "bodily harm" to force a child to write with their non-dominat hand.
Writing with ones dominant hand is crucial, because we build bridges for our thoughts when we put pen to paper.
It's nice to express oneselve in written words. Doing it with your non-dominat hand can prevent that and often feels like being locked-in. You are aware of your own intelligence, also in comparison with other people, but have a much harder time to express yourself, way harder then neccessary.