Incorrect Slap Technique?
 

So, from what I gather, the "proper" technique to slap the bass is to have your thumb angled slightly upwards, with your wrist almost in line with the strings.

I play differently. I play with the thumb angled slightly downwards, at about a 55 degree angle, but nowhere near as downward as Flea does it. (Just so you know I don't do it that way.) Anyone else play like this? I've noticed Les Claypool plays like this occasionally, and extremely fast.

I can slap fast, and accurately, with good rhythm. Now, on to my problem: I think my "popping" technique is off. With the way I do it, I thrust my index, and occasionally my middle finger all the way underneath the string at an almost perpendicular angle to the bass body, and pull upwards. Compared to what I usually see and hear, my pops are very loud and forceful compared to the normal method. While this is good and all, I notice that my recovery time after the pop and right before the pop is lacking a bit. Am I doing it wrong? (Still no broken strings yet, suprisingly!)

To me, there is no "proper" technique. As long as you can do what you do cleanly, without hitting the pickups or other strings making annoying rattling and popping noises, then go for it.

There was a time when there was no slap. Was Larry Graham wrong when he started doing it? Nope. There was a time when nobody tapped on the bass. Was Billy Sheehan wrong when he started doing it? Was Victor Wooten wrong when he started doing what he did? Jaco? Stanley Clark?

You get the point. If you are able to use your version to play the songs you want to play, then you are there. Do what works for you.

I slap with my thumb angled slightly downward also, and I actually pop with my ring & middle fingers. I only pop with my index when I double thumb. I don't know if it's considered "correct," but it's precise enough for me to slap on basses with 16.5mm string spacing, and it's fast without hurting my hand at all, and that's what I think matters.

It does sound like you might be digging in too much with your pops. With all 3 of the fingers I pop with, I only get under the string with the very tip of my finger and pull.

I would agree that the right hand in slap is a very personal choice. Take a look at the difference between Marcus Miller, Brian Bromburg, Flea, and Louis Johnson. and Larry Graham. When I slap, my right hand is nothing like any of theirs. The important thing I have noticed though is that there seems to be a staight line between forearm, wrist and hand. Most of the movement is with wrist and hand. The forearm rarely makes an in and out movement. That is, it does not pull as a whole away from the bass from the elbow (that much)