It's true that Rule 2 defines a "passer" as a player who throws a forward pass. But the newer part of the RPS rule, 9-1-9-b, prohibiting forcible hits at the knee or below, does not require that the opponent actually be a passer, or even become a passer later. He needs only to be in a "passing posture," which could include a stance for throwing a screen pass that is backward instead of forward.
Surprisingly, the penalty enforcement for this flavor of RPS is subject to the same tack-on enforcement as the "traditional" kind, even if a forward pass play does not end up happening. See the unlabeled paragraph following 9-1-9-b at the bottom of page FR-90 (2017 PDF-only rulebook).
All absolutely true, and not offering argument, just realistic discussion. An A player is not often in a "passing posture" to make a backward pass. Even though this rule language does not discriminate between a passer and a player that throws a backward pass, it is clearly designed for those situations in which a forward pass is being attempted (or at least contemplated). A low hit on an A player that throws a backward pass from a "passing posture" is certainly possible, but the chances are very, very low. But, if it does, the penalty is already maximized.
The most likely scenario is the A QB that takes the snap, then turns and throws, from a passing posture, a slightly backward pass to a teammate out wide. The teammate, then, throws a forward pass. While, or after, the QB is in his passing posture for his backward pass, a B player contacts him forcibly below his knees, with no attempt to grasp the QB with his arms (not blocked or fouled into the QB). By the penalty language, this would qualify to be enforced at the end of the last run (if the last run ends beyond the NZ, and no change of team possession during the down).
Other backward passes are not going to qualify, because they won't be made by an A player in a passing posture. Those personal fouls by B could only be enforced from the basic spot related to the run during which the foul occurred.
While we're discussing, let's talk about fouls against a B passer or thrower. The rule addresses the passer, and the passer is a player that throws a forward pass (Team A or Team B). Some time back, the roughing the passer rule required to the pass to be legal, but Redding changed that, to protect Team A passers that intentionally ground the ball, get beyond the NZ when attempting a pass, etc. A forward pass by B is, of course, illegal, but the passer would have the same protections as an A passer that throws an illegal forward pass. Similarly, a B player that throws a backward pass would have the same protections as ANY other player that throws a backward pass. In both cases, however, the penalty could only be enforced at the Basic Spot for the running play during which the foul occurred. (NOTE: If the pass is illegal, the play type is a running play, not a passing play.)
No need to worry about making a judgment on low hit on a B passer, because blocking below the waist is completely illegal by both teams at that point.
Robert