The name comes from drawing silk from a cocoon — pull too hard and the thread breaks, lose tension and it tangles. The correct touch requires consistent spiraling pressure that neither grips nor releases.
In practice, silk-reeling means the body moves in continuous spirals: when you raise an arm, force originates at the foot, spirals up through the leg, through the hip and waist, through the shoulder and elbow, to the fingertips. Every joint participates.
This is what makes it so different when you receive it — the force has nowhere to brace against, because it doesn't come in a straight line.
Grandmaster Feng Zhiqiang made dedicated Silk Reeling Exercises the foundational practice of his Xin Yi Hun Yuan system, done before any forms work.