This specific video shows/demonstrates how much finer you can grind the coffee when you slowly pre brew with a slow flow rate on Slayer. The finer ground coffee results in a (generally) sweeter more balanced coffee with much more body.
The slower flow rate (through the needle valve on Slayer) is completely different than staged or variable ‘pressure’ spoken about and found on other machines. If you can visualize… a garden hose and a fire hose are both are at 5 bars of ‘pressure’, yet the garden hose will fill a swimming pool much slower than the fire hose due to the slower flow rate.
When you make a coffee on a Chemex or V60, you slowly add a small amount of water to the coffee to allow it to bloom where the coffee is expanding due to the release of C02. Only after full saturation and the bloom falls do you continue adding water to the coffee and begin your extraction. If you were to make a coffee using this brew method and another coffee where you simply poor the same weight of water onto the coffee (without the bloom) the two coffees will taste COMPLETELY different. This is basically the same extraction principle when pulling a shot on Slayer…slowly add the water to the coffee allowing it to bloom and achieve full saturation – then go to full pressure to actually extract the shot. On Slayer, you use the needle valve to control how slowly/quickly this water is added during the first phase of the extraction. The needle valve is like a variable giggleur plus we use a 0.7mm giggleur for the 9 bar extraction. No other machine manipulates the flow rate in this way.
Again – this is completely different than other machines which simply vary the pressure through a single giggleur installed in the group head. Yes, you can adjust the “pressure” – but even at 0.8 bars the flow rate is very fast resulting in the coffee becoming saturated quickly. If your V60 recipe called for 25 grams of water for the bloom – you don’t quickly dump 25 grams onto the coffee or somehow add 25 grams of water at a low pressure…this would be ridiculous.
Why are we going to the effort of manipulating temperature, flow rate, pressure, grind? For us its to ultimately change the taste of the coffee…to find every possible nuance and bit of goodness that the coffee has to offer. Slayer was created for a single purpose – to flavour profile the coffee, and to do it simply and elegantly with as few bells and whistles as possible.
When you add the water slowly (during pre-brew) and adjust the grind finer it changes the taste and texture of the coffee. As I explained above, you can only grind super fine when you add the water very slowly during the first phase of the extraction (full saturation). If the water is added too quickly the coffee will choke and the extraction can take easily 1-2 minutes. Grinding finer results in much more body/viscosity as well as balance in the cup. You can (generally) increase sweetness as well as easily muting acidity depending on when you engage 9 bars or reduce the flow rate at the end of the shot. And since every coffee is different you can pull shots anywhere from straight 9 bars all the way to a very restricted flow rate for pre-brew. Once you understand the general concept of how these changes affect the taste of your coffee it is very simple to dial in your coffee – usually within 5-6 shots.