Pelvic tilt -- hmmm -- maybe have someone sneak up from behind and goose you?
Actually, in my experience, the problems people have with Samba and with Latin hip action in general is not a lack of flexibility as much as it is a lack of coordination. Even with a limited flexibility, if the coordination and timing of the various body parts is correct, the dancing will look quite good.
The Samba action itself is pretty complicated and difficult to discuss in writing, but in a nutshell it's a combination of a bounce and latin hip action. The bounce is generally absorbed in the knees so that the overall level of the dancer doesn't actually change much. This point can be illustrated by the following exercise:
Stand with your feet together, flat on the floor, legs straight. Begin to lift your heels while at the same time bending your knees. Whereas lifting your heels would normally cause you to rise, bending your knees would normally cause you to lower, and the two actions when done simoultaneously and timed correctly cancel each other out.
So when people tell you that the "bounce" action of Samba is absorbed by the knees, this is the action that they are describing. It usually happens on the "a" count immediately prior to the downbeat, and tends to cause the pelvis to tilt forward.
Now in addition to tilting forward, the hips can also push out to the back, to the side, and rotate around much in the same way that they might in a Rumba or a Cha Cha. This usually occurs on the downbeat, when the weight is fully transferred to a step. The action of one bent/one straight leg causing the hip to tilt is the same as Rumba, except that in Samba, the leg you step onto is the bent leg.
That's a pretty rough overview, but it should give you an idea where to start. If it's not quite what you were looking for, feel free to ask a more specific question.
Sincerely,
Jonathan
