OK here it is then. I'm thinking something like this.
In Season 5 Spike can't hit Buffy without activating his chip. In Season 6 that changes. If he hits Buffy in season 6, his chip won't cause him pain. The chip still functions, so Spike comes to the conclusion Buffy has come back to life as demon. Tara does research, and tell us "No, that's not what happened". So what did happen?
In S-7's Get it done, we learn the Slayer and the slayer-line is imbued with demon energy. I suggest that's what Spike's chip detected, and that's what changed in Season 6. After Buffy's first death the demon energy left, and passed into Kendra, taking with it all functions governed by that energy. Only power woven into the fabric of Buffy's original body remained.
In Buffy's second death the original body is destroyed. Willow magically rebuilds it. In the Buffyverse magic can have consequences. In this case Willow has built a second slayer imbued with the slayer's demon energy.
Let's flip to The First for a sec. The First is not corporeal, yet it can effect transmissions and communication to this reality. The First is energy. The First is evil. In this way you could refer to it as dark. The First is dark energy. Could you not also refer to demon energy as dark energy? So you have 2 similar energies.
The ultimate plan of The First in Season 7 is to become corporeal. It wants to be able to enter any human on the planet, and in that manner take corporeal form. It tells us this. So how will it manage the deed?
Beljoxa's eye tells us the fact Buffy is alive has created a disruption in the slayer-line The First can take advantage of.
At one time dark energy could only be passed through the slayer line to single, specific focal points.
Willow's spell has changed this. Dark energy has now been passed in such a way it can now exist in two places at once.
If the First can find a way to do it, it can pass dark energy and therefore itself into any person, anywhere. It only needs a catalyst.
Enter the scythe. The scythe can be used to alter the way dark energy is transmitted through the slayer line. We know this because when Buffy is talking to the Potentials in Chosen Buffy tells them -
Willow is going to use the essence of the scythe to change our destiny
Also when Willow does the spell, just after she says "Oh my Goddess", we see her gripping the scythe, the scythe glows, light shoots upwards from the center, and the scythe makes that twangy, zing noise it makes when it strikes something.
So I'm going to Hypothesize the First needs the Scythe to transmit itself through, into this world, in a manner in which it can affect a corporeal state by entering any person it wishes.
First it must kill all slayers and potentials though, because if it doesn't the scythe will direct the energy to specific targets. The First wants to go anywhere it wants, not just into specific humans decided on by a previous spell.
It also needs a slayer. We know this because Willow must be in Principal Wood's office to do the spell. Location is important. The first thing Wood does is direct Willow do his office. Buffy makes a point of saying to Willow "You'll be directly above the Field". In other words, she needs to be "above the Field" -where the Slayers are - to affect the spell.
So the theory goes like this. The First wants to become corporeal. It knows it can do this once 2 slayers exist in the same place, at the same time, because that means dark energy (itself) can now be transmitted into multiple targets through the use of the scythe, and a slayer.
/
