Jelqing at flaccid, you can go crazy. Nothing will happen, aside from mild skin abrasion. Think of dry humping.
When you're transitioning into semi erect and full-erect jelqing, pay attention to the tight feeling of overpressurizing internally. When you jelq with both hands and go full speed, the blood volume may not have enough time to leave before more blood is being pressurized into the penis. This causes the penis to overexpand, similar to being overpumped inside the penis pump. Blood spots or blood blisters can form. This happens when you trap the blood using one hand and use the other hand perform a full jelqing stroke.
A good erected jelq should be a 3 to 5 seconds full stroke from the base of the penis to the base of the glans. The angion method is to perform a 1 to 2 second full stroke while semi-erect at 80%. This creates forward fluid pressure into the glans, and that second when your hand resets from the base of the glans back to the base of the penis allow the penis to be free for that second, the blood will have enough time to cycle out before new blood cycles in. It mimics the natural vascular constriction to trap blood in the penis.