Question about fretting hand positioning for standard drop 5th riff

[u84six]

Hi,
I'm just curious about different fretting hand positions for standard riffs. I don't know if I'm doing it right or wrong, and the standard drop 5th riff (if I'm even saying it right) that most bass players do always causes me to struggle and lose my hand positioning.

For example, you're at the 5th fret playing against a D chord with your hand positioned on the A string (standard tuning/ 4 string), and you want to do a quick drop to an A note on the E string and back up to the D note. I'm sure you all know the riff. Which finger do you use for the E string and which finger for the A string?

[RiddimKing]

I use the same finger. It takes a little practice, but you sort of pivot the finger down to the E string and back to the A. I'm pretty sure that's how most bassists do it...although, as with most things, I could be wrong here too...

[u84six]

There's no way I can do that with a fast riff. I definitely need to either use first and second finger or 3rd and pinky. My question is, is the first finger the one on the root or the fifth? With ring and pinky, is the pinky on the root or the fifth?