Emulator

12345678 rl 16 -> 56781234 wimi 6, 15 -> 06780000 (678 = 3FF).

Therefore unscrambled: a mask 3FF is applied to 678 = 000003FF. It's correct, as it takes the rotation into account. You just did the mask. Using the rotation and mask is good because you can look at the original hex and immediately know what's being masked.

Insert what 10 bits? Isn't the helper consistently showing the what and not the where to?
What would you expect rlwimi r6, r7, 20, 6, 15 to show, the same 0x03FF0000?

I could be wrong here, but it seems to be consistent with what it's showing.

oh, yeah, I guess that's what I'm thinking of. I'm expecting the where to and not the what. insrwi is just the simplified opcode for that rlwimi, insert right word immediate. I'm expecting it to be 0x03FF0000 cuz it overwrites those bits in r6 vs what it takes from r7.