We start with three of the most basic harchakot between husband and wife while the woman is a niddah: kalut rosh, passing objects and throwing them.

Kalut rosh — before any other detail

While the woman is a niddah, a couple must avoid kalut rosh, which essentially means levity and a lack of seriousness.

This is the harder concept to explain and the one most often forgotten, which is why I open with it. The difficulty is that almost no specific actions are classified as kalut rosh. The couple has to use common sense to determine what is fine and what is not.

Avoiding kalut rosh means avoiding conduct that carries, or activities that lead to, levity or flirtation. Sometimes games produce it, sometimes not. It is highly personal.

Rav Uri of Mevaseret Zion says that "kalut rosh" means taking Torah values lightly. What he means, most likely: if something feels intimate, even if it is technically permitted, refrain.

Passing objects — the prohibition and its reasons

Since a husband and wife are forbidden all physical contact, they may not pass objects to one another, lest they touch inadvertently in the course of the transfer.

A further reason — from the story of Yosef HaTzaddik

Rabbanit Lynn Finson teaches a further reason: Yosef HaTzaddik. When Yosef fled from Potiphar's wife, he left his garment in her hand, even though it could serve — and did serve — as incriminating evidence. Why leave it? Because he was at the edge, barely able to overcome his inclination, and he knew that feeling even the indirect contact of Potiphar's wife, through pulling the garment from her hands, could be stimulus enough to make him sin.

Carrying something heavy together

For a very heavy object that neither the husband nor the wife can move alone — a heavy stroller that has to be carried up stairs, say — most poskim hold that a husband and wife may move the object together, taking care not to touch.

Still, Rav Moshe Feinstein and Rav Wosner rule stringently: even then they may not hold an object simultaneously.

Handing over a baby

If a baby needs to be handed over and is old enough to reach out to the other parent, it is permitted to hand the baby over.

Some hold that this may be done only where there is a need, such as when the baby is crying and wants to be held by the other parent.

If the baby is not old enough to reach out to the other parent, then where it is safe to do so, the baby should not be handed over directly but placed on a bed or the like. Where there is no safe place to put the baby down, handing the baby over directly is permitted, though care must be taken not to touch.

Throwing objects — the Ashkenazi–Sephardi dispute

Ashkenazim

Throwing objects from one to the other is also forbidden. Some hold that throwing on an up-and-down trajectory is permitted, unlike a horizontal throw.

Sephardim

Most Sephardi poskim hold that throwing is permitted if done without kalut rosh.