I am assuming that you know the technical ISTD technique but I wanted to point it out anyways just in case.

In Waltz the rise starts at the end of step 1 and continues through step 2. And actually continues through the beginning of step 3.

So to answer your question in a technical way the energy will come from the compression in your legs on step 3 and then into the beginning of step 1. Once you have committed weight into the right leg (step 1) you can start your rise - but just like you mentioned do not rise too quickly but rather through the 2 beats total rise (end of 1 to end of 3).

I hope this helps. If you need a non-technical response I can try:).

Simeon Stoynov