The silver piece is bolted to the bottom of the seat stay and the top of the dropout, and can be unbolted and removed to create a gap sufficiently large to permit the belt to pass through. The chain stay is solidly connected to the dropout and is not affected. The chainstay must transmit bending and torsional loads and needs to be strong and continous, but the seatstay only carries axial loading such that a bolted connection to the dropout will not affect its capacity to carry its loading. In fact, some frames simply bolt the seatstays at the top to the seat tube, such as my old Nishiki rigid MTB. This is why seat stays are typically made of smaller diameter tubing than chain stays.