One of the most important aspects of American Style Samba is the "knee" action. American Style "Latin Motion" (Cuban Motion) uses the concept of having one straight leg and one bent leg. Samba is a little different. The Samba timing, (Ah, One, Ah, Two), results in:Ah One Ah Two
When weight is placed on the "flat", the foot goes to a flat and the knee bends slightly, at the same time the knee on the free leg (in ball position) bends slightly. Normal Latin motion has one straight leg and one bent leg. Samba has two bent legs at the same time (one foot "flat" weighted and the other foot in ball position - no weight).
As in all dances, it is very important that both legs are used at the same time to supply power instead of one leg pulling or pushing the other leg. Each leg moves it's own weight (no free rides!). The bending of the knees accommodates this type of movement. During the "ball" foot movement , the free foot closes "flat" but the "ball" leg does not do all the work. Most of the power comes from the bent leg of the foot closing "flat".
Bottom line: Both knees are bent when your weight is on a "flat". Use both legs at the same time to move your weight. Mastering these seemingly simple actions will take the panic out of your Samba and improve it dramatically.