The positioning required to make this work is very specific; normalized setups are described in individual strats.
In every case, Grapple is used at the end of the unmorph animation (a 2-frame window) to get Samus stuck grappling in a standing position.
By doing this while pressed against a Grapple block and grappling at the very top of it, Samus can get clipped inside a Grapple block by a pixel (or sometimes two, for a block on the left);
In this case, Samus will remain standing after releasing Grapple;
jumping (to force crouch) and firing an angled grapple shot will then result in glitched grapple hanging.
See also: Tech Page