b:19 is a quad full wave rectifier. The module mirrors all voltages below 0V into the positive range. The inputs are normalled to each other in a loop - with no signal plugged in, input 2 takes its signal from input 1, in3 from in2, in4 from in3, and input 1 from input 4. This way, the module also acts as a 1:4 or two 1:2 multiples.