Hi Xristo, if you search the forums here at Thunder’s a lot of people have described the ‘tight string’ feeling when they do a manual stretch that you are expierencing. This is believed to be the septum (middle part of the tunica that seperates the two CC’s Copora Cavornosa) of the outer shaft/tunica that you feel. It make take a week or two or it may take a few days but the way to loosen the septum which is the ‘tight string’ as you feel it is either do the manual stretch called the V-Stretch OR for more advanced people in PE to do fulcrum hanging SO or SD.
I recommend you just start practcing V-Stretchs by pressing gently with your thumb all over the length of your stretched out shaft. Practice this for a while and learn what the feelings feel like and hopefully it will loosen up soon so you can get furthur gains from the shaft/tunica, and even lig gains if you press down near the base and hold the stretch for a while I believe… Cheers.