If the source register's MSB (in SRC1) is set (the 2s-complement integer value is negative), the location where the core will execute the next instruction is pointed to by the destination register (in SRC2). Prepare it in advance !
The Imm16 field is ignored because the negation field already provides with a XOR.
See also the other instructions of the JMP group : JMP, JZ/JNZ, JO/JNE/JNO/JE, JNS