What must F1 do after assuming a pitching position according to Situation L?

In the context of Situation L, after F1 assumes the pitching position, the requirement to step backward from the plate is crucial for maintaining the integrity of the pitching process. This motion may serve several purposes, such as establishing proper spacing and preparing the pitcher for an actual throw to the batter or a base. This step is often used to signal readiness and helps define the boundaries of the pitcher's actions.

In contrast, the other options might imply actions that are not defined as mandatory. A preliminary motion could be seen as an unnecessary distraction or could lead to potential violations if misinterpreted. Pitching immediately without any preparatory action could lead to a rushed throw that may not be in line with the established rules of play. Likewise, throwing to the bases without the correct context or reason could be deemed an inappropriate response to the situation at hand. Thus, stepping backward is the action that aligns with the prescribed rules following the assumption of the pitching position.