One of the most important qualities of the Waltz is its Rise and Fall.
In order to understand the Rise and Fall, a little understanding of what your footwork should be is necessary. Taking simply the first 3 steps of a natural turn as a general example, gents step forward on the Right with a heel step, step side onto a toe with the Left, close together onto another toe with the Right foot, and then before moving into the next step, lower the Right heel and release the Left knee, preparing you to take your next step. For ladies, you step backwards through your Left foot (ball-heel), side onto the toe of your Right foot, close your Left foot together with your Right foot and lower the Left heel (popping the Right knee) before moving on.
The first step is taken into the "down" part of your rise and fall. Like a wave, it should move smoothly upwards as you step onto your toe for the second step, reach the peak of the wave as you bring your other foot together (still on your toes), and begin to "fall" as you lower through the supporting leg and gather the force to move you into your next step.
A large amount of power can be drawn from your "down" steps as they push into the floor and allow you the best point of being grounded in the whole sequence. This can be used to add emphasis to your movements - making them more dramatic and confident looking - and to really move through the steps and cover ground, if that is your decision.
The top of your rise is a great place to suspend timing for a breath in order to play with the rhythms of the music or delay the second step by a partial beat - adding interest and variety to the traditional even rhythm.
Practice your rise and fall as it is an important part of not only the Waltz, but a lot of the other Ballroom dances as well. Strength in your ankles, calves, and centre is needed to take advantage of all that Rise and Fall has to offer your dancing so be sure to do exercises that help to give you control and endurance.