This comes from a skier of 40 years trying to finally get it right and is a technique question. In the diagonal stride, when the weight is transferred onto the the glide ski should the center of gravity be over the heel, over the ball of the foot, or in between (meaning, do you land on your heal, ball, or in between)?
The short answer is ball of the foot. I would discourage the a focus on “getting your knee over your toe” as often alluded to because you can have your knee over your toes and still have your hips too far back, in fact, that would be the body’s intuitive response. This will transfer your weight into your glutes. For a visual this would be as in a “sitting on the pot” position. Overall that is bad because your weight will be behind you acting as an anchor, not propelling you down the trail where you want to go (forward and with efficiency).
To help you get into the correct position:
1. Start in the basic athletic body position, weight is distributed equally on both skis.
2. Softly flex your ankles, they should be supple, like “little shock-absorbers”, knees should be similarly light and loose, core stable.
3. Gently roll your weight from your heels and midfoot until all the weight is in the ball of the foot by way of pressing your hips forward. The weight transfer should be initiated by the hips with the legs, knees, ankles to follow.
It may help to watch video clips of the World Cup (like this: https://www.youtube.com/watch?v=9ei-oSh9CyA) and look at their feet as they diagonal stride the climbs. You’ll notice they remain on the ball of their foot, but also watch their hips and how much of the power and propulsion comes from there. More FIS videos of good, efficient xc-ski technique from FIS here: https://www.youtube.com/channel/UCeVCgLDdCHydzipk5G9HeFQ