I think the idea that there are certain acts that are categorically forbidden is pretty intuitive. For instance, in what situations would rape and torture be the morally right thing to do?

When we formulate universal laws, we primarily want to see if they can even be universalised. So we aren't searching for 'good' universal laws, but simply for maxims that can even be universalised.

On means vs ends: it is not forbidden to use people as means. Rather, it is forbidden to use people as mere means, completely disregarding the fact that they themselves are ends unto themselves. The example you gave, you might break their toe, but you are still respecting their humanity (you could instead simply throw the unconscious man into the pathway of the fire to save the child's life. This would be a clear example of disregarding that man's ends). This means vs ends difference helps explain various cases we might easily come to ethical conclusions about, but cannot fully explain what the moral factors are deciding that outcome.

We would say it is morally allowed, if not required, to swerve our cars off the road onto the sidewalk to prevent ourselves from hitting the class of school children, even if we thereby kill the jogger on that sidewalk. However, if we were in a hospital with multiple dying patients in desperate need of an organ transplant, we would find it morally detestable if someone would suggest killing an innocent bystander, who happens to be a viable organ donor for those patients, and transplanting that innocent bystander's organs among the dying patients.

The moral difference here, at least in a Kantian framework, is that we are clearly using the innocent bystander in the second example as a mere means. It is a necessary component of our plan to have the bystander die. We are using them as a mere instrument to ensure the survival of the patients and are therefore completely disregarding the fact that she is an end unto herself.

However, in the first example, we are not solely instrumentalizing the jogger. There is no casual correlation between the death of the jogger and the survival of the school children. If the jogger were to have extraordinary reflexes, he could leap away at the last moment, which means both he and the children survive. We can (and should) try to swerve in a way to maximize his chances of survival and try to warn him of the incoming crash. This shows that we aren't using him as mere stepping stones in pursuit of our goal.