SPX shredding is no half-baked 'NISPOM' or 'DOD' or 'NSA' or 'military grade' 'RAM' shred - those so-called methods are useless and easily defeated by modern recovery techniques.
Gutmann shredding implies both four 'front end' and four 'back end' overwrites; a staggering 27 (twenty-seven) additional overwrites are performed in between. The exact nature and order of these overwrites is not known until runtime and is never revealed. After each overwrite, the data is forcibly written out of cache and to disk.
That's the essence of complete Gutmann shredding, but SPX goes farther.