The EBS alignment is a configuration that provides a response curve having extended bass, but at a reduced level. It will generate deeper bass than a maximally flat alignment and is used to increase bass extension, but is done at the expense of output in the lowest octaves. The response curve of an EBS alignment is shown below:Response from EBS alignment
You can see where the alignment gets its name. Bass response is extended from peaking at very low frequency, and a region of lower output is formed - an "extended bass shelf."
This and other alignments are shown on the post called "Response curves of closed vs. vented systems."