That’s a really good question. Supposedly, 9k+g executes 1 frame faster than 9k. The reasoning is to allow the system some leniency when it’s translating inputs. If you input 9k, the game will wait 1 frame to ensure that you want 9k instead of jumping k. If you press 9k+g, the game recognizes that you want 9k, and it will not wait another frame to register the attack. Since jumping has changed in VF5FS, I’m not sure if this is true now, but it’s a habit borne from previous games. There was some discussion about this in this thread.
So what effect does this have on gameplay? Very little. 9k (or 9k+g) is used in situations where you’re going to buffer it anyway – eg. [elbow (hit confirm, buffer 9k during hitstun) > 9k] or [block (confirm 9k punish, buffer 9k during blockstun) > 9k punish] – and in these situations, the move will come out the very first frame your character is able to move again because the input delay is swallowed by either the hitstun or blockstun.
When does it matter? It matters when you execute the move from neutral. But going absolutely raw with 9k isn’t recommended anyway.
To clarify, here’s Libertine’s explanation: *"The concept is similar to the execution of Pai’s
http://virtuafighter.com/forums/images/graemlins/default/uf.gif
http://virtuafighter.com/forums/images/graemlins/default/k.gif
http://virtuafighter.com/forums/images/graemlins/default/plus.gif
http://virtuafighter.com/forums/images/graemlins/default/g.gif
vs. her
http://virtuafighter.com/forums/images/graemlins/default/uf.gif
http://virtuafighter.com/forums/images/graemlins/default/k.gif
. The execution speeds for both are identical. For the latter though, the system is waiting for
http://virtuafighter.com/forums/images/graemlins/default/k.gif
to be released to confirm that the player isn’t attempting a jump (entered with
http://virtuafighter.com/forums/images/graemlins/default/uf.gif
Hold
http://virtuafighter.com/forums/images/graemlins/default/k.gif
). This will take at least one frame." *