So, as we all know the Vahi shows up in every single story animation. However, out of the 6 bios it only shows up in one. Why only Tahu's though?
Tahu is the only one of the 6 toa who has used the Vahi. The fact that it appears in Tahu's animation hints that a connection is at a point when Tahu used the Vahi. However, he has only used it once: to freeze the six Bohrok Kal.
During this time, the Toa "pooled their willpower." The comic then shows the six Toa surrounded by a blue aura.
What if their willpower, combined with the mask of time created alternate versions of themselves in an alternate universe?
But how would the six alternate Toa reach the alternate universe? The answer is quite simple.
There was something about how the 6 stars need to align for the Toa to be summoned to Okoto. When Tahu used the Vahi, the 6 Bohrok Kal were frozen, and in a way aligned. If they cannot move then they can be connected/aligned, yes? The stars are the representation of the Kal.
But how would the freezing of the Kal result in a gateway to the other universe? The Krana Kal are capable of talking to each other, so if given enough power they might be able to "talk" to another universe. When exposed to the power of the Vahi and the Nuva symbols combined their Krana Kal turn silver.
This symbolised the Krana Kal being overcome with power, and with all that power they are able to "talk" to the other universe.
When the Kal are able to talk to the other universe, they send the alternate Toa through and the protectors believed that they have summoned the Toa.
So, what do you think of my theory?