This slow movement is in the usual ternary format, A-B-A, but the contrast between sections could not be greater.
The A section is in E Major, and it is slow and contemplative, giving the violin and cello the chance to toss pizzicato figures at each other. Its much like a dream; you can just float.
But in the central panel at 5:00, the dream turns into a nightmare. The key moves one-half tone upward to F minor, and its pure horror and anguish, the cry of a man given a death sentence. There has never been anything like this in Schuberts output.
At 7:51 the nightmare disintegrates; everything falls apart in a series of musical sighs. Then from the horror comes solace: the E Major theme returns in fragments as the first violin and second cello play filagree around the theme, which is only hinted at via harmony in the other three instruments. Finally, the A section is played straight.
At 13:50, there is a return to F minor, but only for a second; Schubert is not going to return to that hell again. He ends it with a sense of peace and resignation. His will be done.