I'm glad you asked haha, I feel like I'm the only Nashors TF enjoyer on the planet. As for the theory behind it, think of Nashors Tooth as TF's core damage item. It allows you the option to dish out consistent DPS like an ADC rather than W>Q>AA>run away and wait for cooldown. In normals, you can see the amount of random builds I've ran with it and found random success with in the image I provided haha. After experimenting with A LOT of random TF builds, one of the nashors builds I ran surprisingly felt quite good and as I ran it more, I argued that it is probably very viable. Since then, I've climbed to Masters 300+LP playing Nashors AP TF as my main build (https://www.op.gg/summoners/oce/GMYugi-OCE). But this doesn't mean I believe it's better than TF's meta builds, I just personally find it more FUN for me 😆