In Step 14, the ALU operation is set to add (F=101 does this) and the shifter is told to pass through the result of the ALU unchanged (SH=00 does this).

Step 15 tells the ALU that its next input will be from port 0 of the AMUX, which is connected to the output of the shifter. Steps 16 and 17 strobe this new value back into the A register, thereby effecting the storeback stage.