Hi,
I'm no expert at WILD but I can hold my own with DILD, using WBTB.

Basically the difference is when you get out of bed for a WBTB then you are 'waking' yourself up more, whereas if you stay in bed then you are highly likely to fall back to sleep again.
There is a fine balance in all of this.

Some people look for 'markers' like feeling lighter, more relaxation, hypnagogia, etc. However not everybody gets this, or it can be fleeting. It is important that you don't rely on the 'noise' which may accompany your attempt as focus needs to be away from your body.

I generally start losing my counting anchor!




The above hypnogram should give you a good guide in regard to your REM question.