Goten & Trunks in U7 are born without tails like Cabba, Kale, & Caulifla
Gohan was born with a tail
Toriyama got lazy with drawing tails, so to get away with it,
he wrote that the tails became a recessive trait, genetically,
and that Saiyans born without tails are more powerful
hence we see Trunks and Goten being born with extreme power and easily attaining SSJ
U6, the tail-less Saiyans are said to have 'evolved' that way biologically
If you make the comparison, Trunks & Goten, and any Saiyan in U6 who
doesn't have a tail at birth also 'evolved' that way.
The Evolved U6 Saiyans are Potentially Powerful as the tail-less crossbreeds of U7
So it actually does add up and make sense how easily the U6 Saiyans can go SSJ.
As far as the Ki/orbs/energy being focused in the upper-back,
You gotta remember that this show is based on Far-East concepts of Qi (Chi/Ki) and Marrial Arts at the end of the day. I'm not an expert in that stuff, but I did see a Martial Artist breakdown the real life comparison of what Ultimate Gohan unlocked or tapped into. He also cross-examined what Toriyama said in interviews about Genki, Shoki, & Yuki (they're sub-sects of the entire Ki spectrum).
Much like there's an explanation for Meditation, Ki, and Gohan unlocking Ultimate,
I'm guessing that there may be an explanation for energy being concentrated in the back. Notice that Caaba was trying to make Caulifla mad, and she was easily aggravated, and when it was explained that she had to get mad to go SSJ, she asked him what else does he feel physically or different when he taps into that form... He replies that he feels a tingly sensation in his back.
It's said that Caulifla is a fighting prodigy, so she asked the right question to find a shortcut. Being angry could automatically be focusing energy into the back. So she just skipped the emotion, and just channeled the same route or path anger would have tapped into. Because she's a prodigy, she could teach her protogé how to tap into it too.
There's no Master Roshi of the Saiyan Race.
What I mean by that is, in the Human Race,
Master Roshi is not only one of the Strongest of his Kind, but
He's also a Master who can TEACH. He taught Goku and Krillin well.
Krillin ended up becoming also one of the Strongest Humans,
and Goku has become the Strongest Saiyan, and arguably the strongest non-deity
There's not a Roshi equivalent in the Saiyan race of U7,
one who can teach and who is a master.
Caulifla just might be a Teacher or Master like Roshi is to her underling, Kale
somebody's gonna say "oh but Cabba taught her SSJ so isnt he a master/teacher"
Nah, a master has knowledge & wisdom behind fighting and understands the body.
She asked the right question to figure out which area of her body she has to focus to tap into SSJ. She brought it out herself because she's just that good at fighting.
If Goku, Vegeta, Cabba, Caulifla, Kale, Trunks, Goten, Gohan
opened up a dojo
the best teacher would probably be Caulifla,
and that's the one you'd probably get the best results from
Goku will have you thinking of PTSD events in your life to activate SSJ
Vegeta will be mocking and bullying you to the point of you getting into blind fury to activate SSJ
Cabba will call u childish names and insults to taunt you to be SSJ
Trunks will probably hit you with cheap-shots and tricks to get you aggrivated by his sneakiness
Idk what the hell Goten would do
Gohan will probably teach like how he was taught (so like Goku)
while Caulifla will just train you likeba true master would, and have you tap straight into the source
(*these are just my views, we'll see tonight how Kale becomes SSJ and if Caulifla is as great of a teacher/master that I think she is)