那位帮忙解释一下这个磁化矢量是怎么变换的

From your figure, it seems the 2IxSz will simply evolve under 2Jt. So the final result should be 2IxSzcos2Jt+Iysin2Jt.
If you choose the right t to make cos2Jt=0, then it would be Iy.
i.e., t=1/4J.
I am not sure why you use two Jt evolution. I guess it should also have two 180 pulses of Ix and Sx, respectively. Then it is simply a part of the rev-INEPT pulse sequences, which could transfer magnetisation from nuclear I to S.