In this video my friend Al Kaplon (who works for MLB and is a former AAA/MLB umpire) discusses the “Transfer Rule” as they call it now and what we call the “transfer interpretation”.
Now keep in mind, this is for Major League baseball purposes to some extent because the interpretation you want to use on YOUR ballfield in amateur baseball is exactly what Al is talking about, stay tuned for a thorough explanation.
Al has communicated to me a very good interpretation: Check it out!
It all comes down to “Voluntary and intentional” on the release in any manner. On the play with the catcher. He caught and secured the ball no problem… Any motion he now makes with his glove down to his hand and opening the glove (to throw), THAT is voluntary and intentional… so if he drops it no matter straight down or backwards, its in the act of throwing. The key is more his motions, not so much where the ball goes.
Straight down or backwards is a guideline more for the the play at 2nd base and only when it’s so close you are looking which way to call it.