Seems pretty clear to me.
Sword of Plot Advancement: this plot-related item makes the story advance (the game story is about this).
Second Hour Superpower: this skill-related item makes the character advance (the game controls are about this).
The Laconic for Sword of Plot Advancement: "You need this sword to win". This is somewhat true of the Second Hour Superpower as well, but for different reasons and through different mechanics. No matter what you do, you can't get past a certain point in the game without getting the SoPA; in contrast, you could play a good part of the game not getting or using the SHSp, you will be just quite weak/ineffective while doing so (FFVI's espers are a good example).
EDITed 4 example
edited 10th Mar '11 6:32:10 AM by SilentReverence
Fanfic Recs orwellianretcon'd: cutlocked for committee or for Google?

I've been looking at both tropes, and I have a hard time differientiating between the 2. Can someone help clear this up with me? Thanks.
Cogito Ergo Sum