First, let’s talk about why the downward force on presser foot matters. When the presser foot is lowered and compressing the fabric, it needs a certain amount of pressure on it to both feed the fabric and assure that the thickness of the fabric is not stopping a stitch from forming.
The demo throat plate in the photo above has the feed dog area cut out to illustrate what is actually happening with the stitch formation. Imagine a piece of fabric under the presser foot. When the needle is lowered through the fabric, and reaches its lowest point, the thread is tight along side the needle. Yet, when the needle starts to rise, the thread relaxes and the a loop is formed in the thread.
The point of the hook then goes through that loop and this is the beginning of how a stitch is made. If there is not enough slack in the loop, the hook will miss it and no stitch will be formed. This is often called a "skipped stitch".
If there are multiple layers of fabric this causes further resistance on the thread. So, instead of the thread relaxing and forming a loop, the multiple layers of fabric
will keep the thread tight and no loop will form. Again, a skipped stitch will result. Therefore, really thick seams, heavy duty fabrics or even multiple layers of fabric and batting sometimes require more downward pressure on the presser foot. Extra pressure reduces the resistance on the thread so that a properly relaxed loop can form for the hook to pass through.
To add more pressure or downward force on the presser foot (so as to compress the fabric layers), turn the pressure knob on the top of the machine clock-wise.
This will compress an internal spring adding more pressure on the foot!
On the other hand, if sewing with very delicate material, like fine silk, the pressure will need reduced to prevent the feed dogs from biting and making marks in the fibers. Turning the knob counter-clockwise will relax the spring and reduce the pressure on the presser foot and ultimately the fabric.
What if the pressure knob is screwed all the way down and no more pressure can be added?