it is barely visible in the pic. even in video you can't see it very clearly, but it appears nevertheless.
all forms of braking work by converting mechanical or Kinetic energy of motion into some other form. normally this other form is heat, which is how mechanical brakes work, but in electrical braking we convert it into electrical energy. this electrical energy has to be 'dissipated' some where, so we use resistors for that in this case. hence the name Dynamic Braking Resistor.