The Starting Clamp, used in racket stringing. Compresses to hold tension on the string without damaging it, and also useful for providing a secure grip when tying finishing knots. : r/specializedtools
Tennis starter clip - Extreme Tennis
Protennis starting clamp - protennis-shop.com
LYLA Sports Starting Clamp Stringing Tool for Tennis Racquet Badminton Racket : Amazon.in: Home & Kitchen