You should have tone throughout your body when you are dancing, but you shouldn’t feel tense. There is a difference. In the latin dances, one knee is always bent and the other straight. You can practice standing still with your feet in first position( heels touching and toes slightly apart) in front of a mirror, and practice bending one knee while keeping the other straight and then switch legs until you get used to this feeling. This is a very important part of all of the latin dances. It is also where your cuban motion( hip movement) comes from. In the ballroom dances (foxtrot, waltz, etc.) it is the opposite, you will always have either both knees bent at the same time or both knees straight. The ballroom and latin dances have completely different techniques.