What is the depthway stretching of nylon monofilament net -Nettingland

To make the knot tighter, smaller and stronger, the nylon monofilament net should be treated by heat setting, which includes the length way stretched and depthway stretched.
The video shows the depth way stretched by heat setting machines. The process for different net would be a little different, such as temperature, speed, etc.
The depth way stretched means twice heat setting. Before it is treated by this machine, the nets should be treated by length way stretched heat setting in advance.