[Discuss-gnuradio] anyone help me understand usrp fpga code fragment which count rssi

the code below is in sdr_lib/rssi.v I don't understand especially this line: rssi_int <= #1 rssi_int + abs_adc - rssi_int[25:10];

  wire [11:0] abs_adc = adc[11] ? ~adc : adc;

   reg [25:0]  rssi_int;
   always @(posedge clock)
     if(reset | ~enable)
       rssi_int <= #1 26'd0;
     else
       rssi_int <= #1 rssi_int + abs_adc - rssi_int[25:10];

   assign      rssi = rssi_int[25:10];