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At the center of the sequence the net magnetization is on the —y' axis, and the central t delay rotates it 180° in the x'-y' plane to the +y' axis: 26 4 0. For the second half:

Note that rotations about +x' simply add to the number, whereas rotations about —x' (equivalent to cw rotations about +x') subtract from the number. The t delays are 180° rotations about +z (13) or — z (39), so they move the vector to the opposite side (13 — x becomes 13 + x,39 + x becomes 39 — x, etc.). At the end of all this gyrating, we end up at the —z axis, for a net rotation of 180° around the x' axis! This is our magic pulse: It gives no rotation to water and a 180° rotation to the peaks we are interested in. If we start at —y (26), we end up on —y (26):

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