When you close your eyes and go to sleep, you can see a lot of little fuzzy light dots (they are called phosphenes). When you go deeper in sleep, you can sometimes see faces, landscapes, strange objects, etc. in front of your eyes. Generally, as soon as you notice them, they disappear. That’s hypnagogic imagery. If you don’t notice them, one of them become more stable, you generally loose consciousness and a dream begins.
When you practice WBTB, you’re more likely to see phosphenes (they are brighter and more coloured and textured) and HI. That’s probably due to brain chemistry.
There is a moment, when practising WILD or dream reentry, when you can sort of zoom, or go, into the HI and feel you’re surrounded by the background. Then you are in a LD.