thread inside spring strap broken?

[walrus2]

My newish (2 months) F1's apparently have a black string inside the spring straps, because on one of my fins this string has broken. Is this string important for anything? How might I go about fixing it/getting it fixed?

Thanks.

 

[redacted]

It keeps the spring from being over stretched to the point where it can not recover. With my homemade spring straps, I just replace the strings every few years when one finally wear out.