You lost me. Isn’t “insert arc” and “turn in” the same thing?
I’m just an average club racer, so take this with a grain of salt, but generally one should, if the kart is set up well:
1) threshold brake until you reach the point you want to turn in. It takes a while to be consistent, to not brake too late so that you end up understeering–pushing–through the apex, or braking too soon, therefore having to coast up to the turn-in point. You don’t want to start the turn-in too soon, which could result in a whole host of problems.
2) turn in enough so that you lift the inside rear tire, not so much that you oversteer or bind up the kart, and not so little that the tire drops back down too soon, thereby bogging the motor and / or inducing understeer.
3) roll onto the throttle, not so soon or so hard that the rear end breaks away (oversteer) on exit. This can be confusing because rolling onto the throttle too soon or too hard could also cause the rear of the kart to drop too soon, bogging the motor and / or inducing understeer rather than oversteer. Of course, rolling onto the throttle too little or too late simply costs you time down the next straight.
When and how you roll onto the throttle varies with the charactersitics of the track as well as the characteristics of your motor. Usually, it’s started before the apex, and power comes on as you unwind the wheel, which drops the rear.
Frankly, unwinding the steering wheel and rolling onto the throttle are aspects that overlap, and to me should be considered together, not as separate activities.
Of course, one also needs to consider the turn AFTER as well, because you may need to give up some speed in one corner in order to make the most of the entire lap.
I think one can be moderately successful doing just this, but some drivers are adamant that applying brakes while turning–trail braking–is preferable in some corners, so there is even more room for debate.