Whenever anyone describes using the WILD technique, they always say you have to “grab” the image as it “flies by”… But do you just have to imagine grabbing it? Do you have to imagine seeing your arms? Or do you actually make the physical movements as if you were grabbing something above your bed? Or just sort of focus on the image…?
When you get this “knowingness” that you can grab on to stuff, you already have a “dreambody”, so when you reach after the object (image?) it should be your “dream arm” you use.
As MovieMe says it is like using a dream body to grab the scene and pull yourself into the dream. You shouldn’t be moving your physical body although if things go well it may feel like you are… 
There are many techniques that are considered WILD techniques, not really one specific WILD technique. WILD is the process of moving from being awake to being asleep and Lucid Dreaming. There are a number of ways of doing this and what you are describing is part of one of those ways.
WILD may not always include this step… If you find you are unable to do it there are other methods to try and become lucid that are still counted as WILD techniques. 
I’ve heard that WBTB + MILD is a really successfull mix.
It is.