The planet that rules the sign that a planet is currently in is called its planetary dispositor. For example, a planet in the sign of Cancer has the Moon as its dispositor, since the Moon rules Cancer.
The conditions of the dispositor can influence the planet(s) that it disposes (or rules). So, if the dispositor has a lot of stressful aspects, or is located in a sign where it is in its Detriment or Fall, then the energy passed to the planet(s) in its ruled sign(s) will receive a more stressful version of the dispositor's energy. Alternatively, if the dispositor is well placed (Exalted or in Rulership) and/or is well-aspected, then this stengthens the energy that the dispositor passes to the planets it disposes.