Let me explain my reasoning. I know this to be true but I'll try and back it up.
If i make it a little less complex and compare 'rear brake only' to 'front and rear brake'.
As i'm sure you know a tyre has a limited amount of grip, this can be used for turning or braking/accelerated or a combination of both. With a rear braking kart it is possible to do more turning whilst braking (as the front wheels are only turning, no braking). This is known as trail braking. As a result you can turn later applying gradual increase in steering angle, this benefit is very important to a direct drive kart. As their powerband is in a limited rev range (and rpm is correlated to speed) it is of imperative importance to keep the engine in this rev range so you will wish to shallow out the corner and maintain a higher apex speed and therefore keep the engines in the powerband.
This is the not the case for kart with front brakes (and a gearbox). They can change gears to maintain the rev range. Also with front brakes they cannot turn as well because the limited amount of grip supplied by the tyres needs to be balanced between braking and turning, i.e. to brake as late as possible you need to do as little turning as possible. With the combination of this limited amount of grip and the ability to keep the engine in the intended rev range it makes it faster to turn early and point the kart towards the corner, do all your braking in straight line that do a bunch of turning on the apex and accelerate out.
So if I was to attach a datalogger and compare the two styles of driving if for a second you can imagine the speed trace the direct drive/rear braked kart would look like this;
U
and the gearbox/front braked kart would look like this;
V
To elaborate a little the direct drive kart would brake earlier turn gradually (and only once) and maintain a higher apex speed, the gearbox kart would brake much later have a lower apex speed make two distinct turns (once before braking and once at the apex).
Does this make sense?
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