Using subtool subtractively negatively as a cutter in Zbrush-making hollow and clipping and sealing off parts of hollow sculpture

The method that should work but didn't...
https://youtu.be/bLsOb1NwKLA

The method that should work but didn't...
Select object to be cut and make Dynamesh.
Select object to cut with, set to negative icon (crescent moon shape icon) on the layer, go to polygroup>polygroup as dynamesh sub. Object should turn white (but didn't).
Select top object and merge down.
Re-dynamesh (command click and drag in empty canvas).
Should subtract cutter shape from main shape but didn't work.


The method that DID work...
https://youtu.be/pX5XdYoWPj0

The method that DID work...

Select object to be cut and make Dynamesh.

Select object to cut with (below top object), set to negative icon (crescent moon shape icon) on the layer

Select top object and merge down.

Re-dynamesh but click subtract (near main Dynamesh button).

Voila, it worked!

Making hollow
Have a layer of the thing to be hollowed, and a shape for a hole into it below that.
Dynamesh the top one, and probably the bottom one too.
Set the bottom one to subtract (as per 'The method that DID work', above).
Merge the top one down.
Go to Dynamesh and use to create hollow (I used 10 thickness for a belt buckle)

To clip off and seal unwanted parts of hollow sculpture
To clip off and seal unwanted parts (clipping will leave the inside of the two skins open), create a shape that encompassed all you WANT to keep, (probably Dynamesh it, and the above layer) give the lower layer new shape the mode of what's common to both (intersecting circles icon), merge down, and Dynamesh using Dynamesh's intersecting option next to subtract. You may or may not want 'project' turned on for that. Try both and see which is better.