Rotate onto your side and draw a long straight line from fingertips to toes
Also known as "linetation"
Start the stroke here, from this position, with your hips vertical
In core-driven freestyle, energy moves up your body, pushing you forward.
Look at Michael Phelps (above image).
Notice his position: nicely balanced with a long straight line from fingertips to toes. His hips are near vertical. He is looking straight down. His arm is extended straight out in front of him as his hand feels for water to hold on to. As he bends his wrist, pressure will build on his hand, and he'll start the stroke.
In rapid sequence, he'll quickly twist the high side of his hips down, then his shoulders will come around, his recovering arm enters the water heavily but cleanly and sends energy, lots of it, forward; he'll draw another long straight line, bend his wrist, set the catch straight out in front of him, and start the cycle over again.
He is swimming around his long-axis.
Think of your leading hand at the catch as an anchor (it's stationary, you swim past it). Let your hand glide forward (little finger down), bend your wrist 45° or less, load pressure on your hand and rotate around it; set the catch and swim past it. This is the shoulder-friendly way to swim, the powerful way to swim. As you shift the work down from your arms and shoulders to large core muscles in the middle of your body, hip rotation becomes the source of propulsion.
Torque is a rotating, twisting, circular force generated by the movement of your hips. It, hip rotation, is the big engine of core-driven swimming. It's where all the power is.
Notice the word this author chose to describe the amount of rotation in hip rotation: tremendous, which is something great in amount or intensity.
Three words to remember: tremendous hip rotation.