It's not my era of blues at all, but as I've read it (Sandra Tooze) the breakthrough album with Winter - Hard Again - was Winter's attempt to take Muddy back to the 'gritty' sound he had 25 years earlier, and away from what he saw as marketing-led excursions into things like folkiness and psychedelia. Apparently Muddy was very happy with the results (Winter plays guitar) and said it made his 'pee pee hard again', hence the name