Each kayak differs on its position of optimal turning. That is for you to find.
The rails are the edges sharp or otherwise found on the underside of the kayak/canoe. Flat hulled kayaks may have them within reach under the water. Some older kayak models i.e Rotobats were so rotund the kayak had to be almost tipped over to utilise an edge.
Using the boat rail allows you to turn while moving without using any strokes and using the forward momentum of the boat. The steeper the angle of the kayak [lean] the tighter the turn.
This can allow you to turn tightly into a narrow eddy or to move across the river say through a boulder garden with speed and grace.
The faster you move the hull the steeper you can lean. The extreme if this is to dip elbows in water, when maybe coming into the beach big eddy, how much speed do you need to do this? At the other end is the slight hull lean when say ferry gliding or slightly altering course when paddling down stream and the course correction needs less than a paddle strokes input.
The knees are critical in the lean, see previous post in this series -> https://telfordcanoeclub.wordpress.com/2011/08/01/2-1-there-are-many-patellas-but-this-one-is-mine/