In the Samba basic, the rhythm is 1 a 2. The 'a' is at the 3/4 point between the 1 and the 2. On the 'a' beat, the heels release from the floor, and the hips tilt up. The feeling is like doing an abdominal crunch. On the numbered beat the hip tilts back down.
Natural basic, the abs are crunched on 'a', step forward on the right foot on 1, while straightening the left leg and turning out the left foot. The hips twist, but the shoulders do not. More on Samba Technique.