Here is some overview/procedure and things I've learned while using the bits:
The bits fit into the chuck of a hand drill or drill press, using a silicone carbide paste to grind away the surface of your glass fitting, allowing for a better ground glass to ground glass male to female fitting.
The bits can be used on finished pieces with care or before attaching the fitting onto a piece. The more precise the drill the better, a wobbly drill chuck will make grinding a precise fitting very difficult.
The strategy for grinding the formed glass joint/fitting relies on the preparation of the glass, for example you can shape the joint leaving the fitting extra snug so the male end doesn't fully set flush into the female fitting and grind down to the final fit.
You can also shape the fitting and start grinding from where the female fitting allows for the male fitting to set flush and grind more or less just the inner surface of the female fitting.
It is possible to take a nice fitting joint and grind it uneven or make it loose with improper grinding strategy depending on the initial shaping/tooling of the female fitting in the flame.
I've had to adapt the way I shape the fittings taper in the flame to allow for a better fit after grinding.
When using the powder/grit from HIS use a few drops of water to form a slurry compound to begin grinding or wet the bit and dip into the bag of grit and add a few drops of water adding more grit/ water as needed. This process is a bit messier than the already mixed tube of grinding compound. I suggest too use in a utility sink or cold working area.
When using the tube of mixed compound it’s easier to create the grinding slurry most times without any water and is ready to use out of the tube.
Either way you are looking for a slurry to build at the grinding surface and kept there while grinding. Holding the fitting horizontal while grinding works best if you have a wetter slurry to fight gravity and keep from having to reapply more grit every few seconds.
Don't apply too much pressure while grinding and start slow until you get a feel for the slurry to avoid cracking glass.