So I've noticed the lace is a slightly darker color than my skin, I am very fair skinned. What makes it disappear? I haven't officially attached a unit or had it cut in, but I cut off a section of lace and glued it to the back side of my arm (the hairless side) and it most certainly didn't disappear, it was VERY visible. So how would it be any different on my head? If I had darker skin I could imagine it vanish... but on light skin is it going to be noticeable?

The hair that is attached to it makes it disappear. It acts as a cover.

Never is lace just attached without any hair to any part of your head - maybe a 1/8th of an inch up front but that dissolves into the glue/tape.

No matter what your skin color cutting off and attaching "lace only" is going to be somewhat visible.

Don't worry you'll be fine.

Many people cut the front lace off, right up to the hairline.
Reubin is right, the hairs help to disguise the lace.

Using several thin layers of glue before pressing the lace in to the glue also helps.

Hair Scare,
The lace is translucent which allows the color of your scalp shows through the lace. Once the lace is properly attached so that is is firm against the scalp (no gaps or looseness to cause shadows) it disappears. Please remember that the color of your scalp is not constant. With exposure to sunlight the color changes. Also, when the body oils of your scalp mix with the lace this also contributes to hiding the lace.
Regards,
John
http://toplace.com/

Everything above,, its a combination of the material itself and obviously hair covering it.. the color of the lace, how it matches your skin, how translucent it is, etc.. the pattern that allows your own skin to show through.. the light that reflects off your skin that helps to hide the threads because of the way light bounces back. That is why lace looks better right dead on with light above you, as opposed to off lighting conditions where there is no light being bounced back off your skin and lace.