A jump instruction unconditionally transfers program control to a new location in the program.
The ALU must therefore use the 26-bit immediate address in the instruction to create a new PC address.
This is done by (1) shifting left 2 (to create a proper 28-bit byte address), and (2) adding the upper four PC bits to complete the new 32-bit address.
This new address is then loaded into the program counter as the address of the next instruction to be executed.