I overlaid screenshots of Takoda practicing jumping early in his career so jumps were set at 16″ vs. 20-22″. I think these screenshots show how a dog starting with his front feet UP on a well-placed Mark and driving towards another well-placed Mark at the reward point creates a consistent jumping style over all three jumps. Comparing the take-off spots for theh three jumps, you can see his rear feet are perfectly centered. You can also see how he is loading to stop on the ending Mark and that his back is in a similar position as it would be if loading for another jump. If I had used a placed toy instead of a Mark, his head would have reached forward and down during the last jumping arc, pivoting his body weight forward (like a see-saw) onto his shoulders as he dove like an arrow into the ground to grab the toy. Previous posts show videos of this diving action for a toy.
I can’t think of any reason NOT to use Marks for jumping practice.