Your instructor is correct. The kart needs weight transfer to flex and work properly. Your body is the biggest weight in the kart. By moving around in the seat and changing your posture you can completely change the attitude of a kart. A poor handling kart can be fixed mid-race just by changing your position in the seat.
The seat will be tight when you sit in it on the grid. But remember, you’re approaching 2-3 Gs in the corners so your body is going to compress pretty heavily under load. This means you’ll have some space to slide a couple inches side-to-side in the seat in the corners.
If you lean to the outside of the seat with your body, you create a more direct connection from seat to chassis and help put more load on the outside tire. This will help the outside dig in and lift the inside rear more effectively. Conversely, leaning into the corner will do the opposite.
Posture is one of the most important driving skills to learn in karting. I preach about it endlessly when I coach.
Here’s a couple images that demonstrate leaning to the outside of the seat for more weight jacking to the outside corner:
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