Sometimes, observing the barista's pressing action can also be a decompression thing! Because each barista has a different understanding of flour pressing, they will have different pressing actions. 
Flattening the powder is already a thing that everyone can agree on, because once the powder is uneven, one side is high and the other is low, then there will be a channel effect, which will eventually lead to uneven extraction. The difference in a barista's understanding of the ground is how much pressure it needs to be applied to compact. Two different schools of flour pressingBaristas who think that they can just flatten and compact the powder cake will usually only gently rotate the powder hammer when pressing the powder, and then apply a little pressure, and the whole set of actions is smooth and elegant. Because they feel that no matter how hard they press the powder, the firmness of this powder is not worth mentioning in the face of the high-pressure extraction of the coffee machine!
And if it's a barista who feels the need to use a lot of force to crush the compact, then you may see that all his power is on the arm that presses the powder! Press the powder with a force that seems to lift you up, as if to say, only one of the table and the coffee powder can be flat! Because they feel that the amount of pressure we use to control the flow rate is effective, and the more force, the more uniform the extraction will be. 
So, what kind of pressure should we use to fill the powder? Powder pressing experimentTwo sets of experiments will be carried out using two different types of grinding, coarse and fine, each with a different pressure. See if you really need to control the pressure when filling the ground coffee!
The grinder used in the experiment was Galileo's Q18, the powder amount was 20g, the weight of the extracted target liquid was 40ml, and the scale of fine grinding was 15. The scale of coarse grinding is 25. Evaluate whether the strength of the powder can cause the change of extraction by the extraction time! 
Figure 1 on the left5 scale ground coffee, 2 on the right5 scale ground coffee).Comparison of fine grinding and pressing powder
With 1After grinding 20g of coffee powder on 5 scales and spreading the powder, we used 15kg (normal) and different degrees of pressure and full force to fill the powder! 
The limit of the electronic scale is 3kg, so we borrowed the scale of the neighbor next door, and the weight of the use of full pressing is about 9kg) Use 1The time it takes to extract 40ml of liquid is 30 seconds for a 20g powder cake filled with 5kg of pressure. The extraction time for the pressed pressed powder is 33 seconds, and the extraction time is still 33 seconds to prevent changes in the coffee powder. It can be seen that the finely ground powder cake will have an extraction effect due to the change in the pressing force.
Contrast of coarsely ground pressed powderAlso after the cloth powder, use 15kg and different pressures of the full force to fill the powder. The results were strikingly consistent, with the same extraction time of 20 seconds for 40 ml of liquid (which is shorter due to the coarser size).
Conclusions of the experimentFrom the above experiments, we can see that the difference in the density of the coffee powder will lead to a certain difference in the strength required for the compacting of the powder.
What does that mean? For example, the reason why our fine grinding experiments produce different results due to the different pressing forces is that the particles of the coffee powder are smaller, and if you need to press it to a tight state, you need to use more force! And 15kg of powder is obviously not enough, it requires more force to press the powder to the fullest. 
If we increase the intensity to 2kg a little, then the extraction time will be the same as that of the pressed powder, because at this time, the two powders are already the same degree of firmness, so the extraction time required will naturally be the same!
This is illustrated by the coarsely ground powder group, which has a large gap between the coffee particles in the powder, 15kg of pressure is enough to make the powder very firm, so even if you use all the strength to fill the powder, the extraction result will be the same as 1Therefore, we can properly adjust the pressure required between different thicknesses, the finer the force, the more the force can be slightly increased
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