If I practice for a few minutes one way, I can do both reasonably well. -- relaxed or firm tips. It's certainly possible that some player's tips don't bend backwards, most do, but I don't think we can say that a relaxed tip that's allowed to bend backward (however much) is the only way to play, that would be illogical. Nobody talks about doing pull-offs in the fretting hand while allowing the tip joints to relax, they must remain firm to to get that job done, and no one would suggest that firm tips in the left hand means you're playing with too much tension. My left hand is very fit and I can turn the guitar over and play left handed and have no problems playing alternation and arpeggios with absolutely firm tips. I can do the same thing right handed, but that hand isn't as developed. When I play with firm tips, I feel that the MCP is tensioning the string while on the string, but it's a combination of PIP/DIP flexion that release the string and create the sound with the MCP releasing/extending at that moment. This movement is also what I see most of the great players doing. Interestingly, if I play with completely relaxed tip joints, my hand does feel very relaxed, maybe too relaxed in a way. It feels more like I'm pushing through the string from the MCP and the tip is just inactively allowing the MCP to push though the string by getting out of the way a little. In this case I can get the exact play-relax movement that Kanengieser teaches in his Effortless Classical Guitar dvd. I've seen players who do this movement with some success, but mostly this supposedly pure play-relax pendulum movement seems to be unattainable in the real world and concert players flexion the middle joint much more than this. Also, I've asked concert players about allowing the tip joints to bend back for free strokes and some say they do it and others say they don't do it and some say they do both, that's why I say it's one of the variables in right hand technique but by no means the only way to play.
An eyewitness will often only see what he already believes to be true.