Fill it
The upper black housing, the PEEK plastic ring, inserts down into filled chamber. It compresses/ shoves herb bits at perimeter, seating down and making contact with metal chamber at the bevel point
Pic on right shows load after screwing down upper housing. The load makes good contact with upper screen, unable to shift. If you only fill to the bevel point, you will have slight shifting and slightly weaker hits
I used reducer with capsules exclusively for a year. Preferred neatness and ease. But found I needed to shape upper screen with a nipple protrusion. The reducer's inner ring is not tall enough to press against upper housing screen (S&B fixed this height gap error with the Volcano Hybrid reducer ring -- it meets flush with upper screen, and, they changed upper housing ring to metal from plastic. It is beveled to marry with the bevel on Hybrid herb chamber, so filling Hybrid to bevel line is OK)
Edit: 0.75g
another edit:
@sonti - great that you are reading the manual. One important item users overlook is how to remove and re-insert the metal chamber from orange plastic housing. S&B has diagrams showing how you press up from under to remove, never press down. One or more of the tiny orange plastic retaining tabs will break. Habits die hard, and most folks use q-tip swabs to clean bowls of other vapes while still warm. Do not press down and wipe chamber with q-tip, try to break the habit before you break a tab