I can write a whole chapter of possible ways to swim that more or less use fishlike actions or not, but thats only making things more complicated.
Ok , we Have Dannys extra freedom of forward and rearward movement of bodyparts within the bodytube. The shouldercomples can be shifted forwards and the other side backwards without bending if you got the shoulderflexibility.
Thats a good way to increase arm power and range of movement.
The shoulders can also be rotated relative to the other bodyslices, but that goes against moving the whole upperbody as one unit. The tilting of the hip causes some spine bending, thats a bit of a bordercase if you want to stick to no bending allowed-
If I understand Suzanne right, a good kick goes together with some undulation.
hip drive to me implies rotation coupled with forward movment.
you're talking about a degree of undulation which will always be present of the kick is to provide forward motion.
Shelly Taylor Smith has an almost straigt leg kick, but I agree this style cant be used by everyone with the same effect. The straight leg lick can be a source for rotation, but for most not the most effective way to extraxt forward movement.
In principle the legs should get some traction on the water to help the rotation, right?
When Shinji kicks he gets a litle bend in his bodyline. He sort of kicks his butt to the surface.
Is that the sort of undulation you mean?