The controlled rise and lowering of the body through ankle, knee and ball of the foot. Four of the five standard dances live on it — only the tango deliberately stays flat.
Rise & fall is the difference between marching and swinging: an S-shaped wave running from the foot through the knee into the body. It is built from three parts — body rise (knees and hips extending), foot rise (coming up onto the ball) and the fall back down. The most common beginner error is lifting with the shoulders; the movement starts from below and the shoulders simply travel along quietly. Just as often the fall gets misread as dropping — it is active, on soft knees, otherwise your balance tips and your joints take the hit. Many teachers time the waltz as “down — up, up — down”. What gets missed easily: rise & fall is a long, flat wave, not a lift — visible bouncing means too much amplitude. And the tango has none at all; importing it there gives you away instantly.