Hadoop Mapreduce Cartesian Product How Can Record Reader Send Data To Mapper

I'm new to Hadoop and currently I'm learning mapreduce design pattern from Donald Miner & Adam Shook MapReduce Design Pattern book. So in this book there is Cartesian Product Pattern. My question is:

1. When does record reader send data to mapper?
2. Where is the code that send the data to mapper?

What I see is next function in CartesianRecordReader class read both split without sending the data.

Here is the source code https://github.com/adamjshook/mapreducepatterns/blob/master/MRDP/src/main/java/mrdp/ch5/CartesianProduct.java

That's all, thanks in advance :)