Ascension is a form of shifting that can be performed by any strong person, regardless of species, though it is very hard to accomplish without working with Ki. A Limit Break, while not a shift, is still related.
Ascension
What is Ascension?
Ascension is when you Ascend to a whole new level of power. A Limit
Break is a temporary boost which runs out but Ascension is something
that will be with you until you power down.
The best way I heard it described was on the old Ki Warriors site:
Imagine you are a container (a bucket) that normally holds 5 gallons of
water (this will be your Ki). When you use Ki your water level goes
down, and when you power up it goes back up but never over 5 gallons
since that's all you can hold. A Limit Break is something that allows
you to hold 6, 7, or even 8 gallons of water (depending on how strong
you are). However you can't get those bonus gallons back by recharging
so once you use them you're back down to a maximum of 5 gallons.
Ascension is like getting a whole extra bucket so now your maximum is 10 gallons and you can keep recharging to that
until you power down from your Ascended state.
What does Ascension feel like?
It's hard to describe. You feel powerful but not like with other techs.
You don't just feel physically and energetically powerful, you also
feel confident and in control and you know you will win.
Limit Break
What is a Limit Break?
Obviously, it is when you break through your supposed limits. These are
often seen on those shows with "incredible videos." Tiny woman lifts a
car, firefighter keeps going into the building to save someone even
though there's not enough oxygen for him to physically keep breathing,
etc. A Limit Break is when emotion and determination trigger extreme
focus and strength (the Limit Break itself) allowing you to go beyond
your limits, though only for a short while.
What does a Limit Break feel like?
There's not much "feeling" involved. You are incredibly focused on the
task at hand. Looking back you realize you were moving faster and your
attacks were stronger but right there during the Limit Break none of
that registered. All that matters is winning.