"Shoal keels can be a little more tender initially "
What does this mean?
Sailboats are often referred to as being "tender" or "stiff" depending on how quickly, and how far over, they heel. If a boat resists heeling at all angles of heel, even small ones initially, and does not heel much beyond a stability point at a low to moderate angle of heel, it is said to be "stiff." If the boat heels easily at low angles of heel, and if it tends to sail fairly well heeled over most of the time even at increasing angles of heel, it is said to be "tender."
When someone says a boat is tender initially, it means that the boat heels easily at zero to low angles of heel, but as it heels further it reaches a stability point where it becomes comparatively "stiff." In other words, it
resists further heeling beyond that stability point.
A boat with a shoal draft and winged keel may be tender initially, but then become stiff when the stability point is reached. Thus, as the boat heels, the "wing" on its keel becomes deeper in the water than when the boat is fully upright, thus increasing the "functional draft" of the boat as it sails over a shoal-draft one w/o a winged keel. So, people have told me, not being a yacht designer myself.