I am not sure I'll be able to describe dysgraphia as well as you've just described hypermobility but here goes:-
Dysgraphia is a specific learning difficulty. It is a neurological disorder in the dyslexia family. Dyslexia relates to reading, dysgraphia to writing and dyscalculia to number work (apparently the numbers just float about in front of your eyes). Dyspraxia is about gross and / or fine motor skills. There are other disorders within the dyslexia family, but these are the main ones.
There are about half a dozen variants of dysgraphia. The combinations are made up:-
Can't write legibly at all
Can't write legibily, except when copying out
Can't spell.
DS has the type where he can't write legibly under any circumstances, but his spelling ability is unaffected (actually his spelling has better than most adults since he was about 8).
The symptoms of dysgraphia are:
unable to write on the line
unable to form certain letters to look ok no matter how slowly he does it or how often he practices, even by joining the dots. (S, C and G are the worst).
odd pencil grip - more like he's holding the pencil in his claw than in his hand
slow writing
wrist pain when writing - even after just 10 lines
unable to think and write simultaneously
The last symptom (thinking and writing simultaneously) is the killer when it comes to school work. As I am writing this, i am think what my next sentence will be, and I am holding the structure in my head. DS can't.
DS is very clear thinking , he's insightful and he has an exceptional general knowledge, which should add up to some very exceptional answers in reading comprehension, essays, geography, history etc. Except he can only do this verbally. As soon as he writes down his answer, he boils it down to as few words as possible, cancelling out everything not directly related to the question. He'll make blank statements and omit the justification, even though he can easily justify the statement verbally.
From the reader's point of view, you are almost glad he does this because reading something Ds has written is very hard work. Even my birthday card requires some effort (and I can make an educated guess what it will say before I even open it).
Special pens and writing slopes both help but do not solve the issue. Mainly they reduce the wrist pain so DS is willing to carry on a bit longer. The main solution is to touch type. The school are very encouraging of this. They want DS to type his work where possible. I've given him a keyboard for his ipad and a PC at home. His typing speed is still slow, but already faster than his writing speed. Moreover, he can type and think at the same time so the quality of his work is better.
However, DS - who has just turned 13 -is exceptionally embarrassed about his disability. He's at pains to hide it from his peers. Sometimes he cries about it at home. The bottom line is he'd rather understand everythign in class, find it all exceptionally easy, but turn in written work that is nothing special at all.
He's in the top set across the board. I had his IQ measured as part fo the dysgraphia diagnosis, and its off the scale in places. However, I don't know how long DS can survive on turning in the sort of work he does before he slides to the bottom of the top set and then down a set (which will devastate him).
He needs to overcome his fear of being seen to be different and start typing in class. He understands the theory behind that statement but he can't bring himself to actually do it.