Yes, you have the right idea. It’s a bit more complicated than that (it always seems to be ), so I’ll add a bit to what you understand already.
The activation of the legs - and this applies to most body parts - is to stabilize and align. I realize these are general terms, but it helps to understand this when you’re learning about the fundamentals, such as core stability. When you are standing in mountain pose, you want to feel your legs squeeze (lightly) in, press (lightly) out and lift (lightly) up toward the crown. I emphasize ‘lightly’ because this is not about power, it’s about stability and length, and the purpose is to create the feeling of length and lightness in the body.
Have you done Rachel’s Empower Your Boundaries? If you haven’t, it is the best class we have for teaching you how it feels to engage the legs. The feeling that you get when the legs are together can then be applied to any pose, whether they are together or apart. You still want to feel the light squeezing in, pressing out and lifting.
Let me know if that makes sense and if you have any further questions.