Jefferson Curl question

I recently showed JC to a friend, which answered to me with this video: https://youtu.be/04Vu32iZ0iY (it is in Spanish, but it says that pike exercises should be done with APT, and avoid hunching). 
I know that in Pike APT must be held to keep the stretch on the right muscles, but I do not know the difference with JC. I know the importance of going vertebrae by vertebrae on JC and keeping the weight close to the body, but I could not explain to my friend why JC is not bad for lumbar or spinal kyphosis.
Just out of curiosity, Thanks!

If you overloaded the JC curl it would not be so clever. The idea I believe is to slowly strengthen often neglected areas that are still a natural movement and to help improve range of motion. 

2 hours ago, MARIO LOPEZ RUIZ said:

I recently showed JC to a friend, which answered to me with this video: https://youtu.be/04Vu32iZ0iY (it is in Spanish, but it says that pike exercises should be done with APT, and avoid hunching). 
I know that in Pike APT must be held to keep the stretch on the right muscles, but I do not know the difference with JC. I know the importance of going vertebrae by vertebrae on JC and keeping the weight close to the body, but I could not explain to my friend why JC is not bad for lumbar or spinal kyphosis.
Just out of curiosity, Thanks!

Let me say that your friend doesn't know what is the Jefferson curl and its purposes 

the JC is a curling/extension spine movement and not a lower posterior chain work. with APT you will end by stressing the long head bicep tendon, something that you should absolutely avoid at this stage of GST., on the contrary, you should curl the spine when you move down and uncurl/extend the spine when you move up and nothing more. the exercise becomes a way to increase the erector spinae muscles in the point of maximum length up to the point of minimum length, this is the long term gives you cold flexibility.