I don’t know if this will help, but……
The following excerpt is directly from the book, Introduction to Human Disease written by Thomas H. Kent M.D. and Michael Noel Hart M.D.
“Vascular Disorders: Vascular changes that lead to thrombosis include acute and chronic injuries to vessel walls (including the endocardium) and turbulent blood flow. The most important of these conditions, including thrombi in leg veins, thrombi on atherosclerotic plaques (gunk in vein) mural thrombi over a myocardial infarct (gunk around heart) and atrial thromi………. Any vascular injury or site of turbulence can lead to thrombosis.”
Now to answer your question in plain english (no pun or disrespect) both. If you jelq to hard, you can cause a thrombosis, which is the condition of having a blood clot; thrombi is the actual clot.
The bigger they are the more damage they could likely cause. If it causes pain, quit. Let it rest and then start back in slowly. You don’t want to hurt yourself, that part of the anatomy is important for several reasons. It would be reassuring to hear from either luvaduds or boxcar 23 as they are both M.D.’s.