I'm an Akuma player, and over the past few days I've been testing out this alternative way of hitting my links.
For instance, for the following BnB combo:
Jump-in, cr.LK, cr.LP, cr.LP, cr.MK xx EX Gohadouken
Note: the link from cr.LP to cr.MK is a one-frame link.
Instead of double-tapping the MK in the above example, what you can do is:
Jump-in, cr.LK, cr.LP, cr.LP, cr.MK+LK (MK~LK input) xx EX Gohadouken
By MK~LK input I mean, you almost-simultaneously press LK and MK buttons with your index and middle fingers, but hit the MK button with your middle finger slightly earlier (1 frame) before the LK button.
If done right, in the training mode with input display on, you should see this:
(second frame) down+LK+MK
(first frame) down+MK
Note: if you pressed the two buttons too fast/simultaneously, you'll see (first frame) down+LK+MK and that's it, no (second frame to speak of). If you pressed the two buttons too slowly, it would have been (first frame) down+MK, (second frame) down+LK.
I've found this method to work more than 90% of the time for me - whereas double-tapping (perhaps due to my poor technique) works maybe half the time.
Update 01-19-2010: Plinking versus Double-Tapping - when is which better?
Recently I read in a Japanese SF4 wiki that whenever you double tap a button, what happens is you're (1st frame) pressing the button, (2nd frame) releasing the button, (3rd frame) second tap of the same button. Two inputs of an attack button in 3 consecutive frames.
If you use plinking or "sliding frame input" (which btw is a loose translation of what the Japanese call plinking; none of that Tsuji-style egotistical nonsense which I wish some mod would remove from the thread title), you basically get two inputs of an attack button in 2 consecutive frames.
The wiki just stops there and moves on to how you can apply this knowledge to your game (source: http://www5.atwiki.jp/koko100/pages/203.html
But if we inspect closer, we can see that if you're someone who can perform both double-tapping and plinking perfectly (or at least, very competently), each technique actually has its own advantages. Theoretically speaking, at least.
For one-frame links, plinking is undeniably superior. With plinking you can either hit the button one frame early (-1), or time it perfectly (0). The margin of error for plinking here is 2 consecutive frames. With double-tapping, however, you either hit it two frames early (-2) or squarely on the dot (0). The margin of error here is still 2 frames, but they are not consecutive frames.
For two-frame links and onwards however, double-tapping (if you can perform it perfectly) is superior. For a two-frame link, the margin of error for double-tapping is 4 consecutive frames (-2, -1, 0, +1) whereas for plinking the margin of error is 3 consecutive frames (-1, 0, +1).
Same goes for three frame links. Double-tapping has 5-frame margin (-2, -1, 0, +1, +2) whereas plinking has four (-1, 0, +1, +2).
That said, personally I find double-tapping to be incredibly to pull off perfectly, and so I stick to plinking for all cases (except for LP link, which plinking cannot be applied to). But if you're someone who can do both, you should test out and see if double-tapping is indeed easier for hitting 2-frame-and-up links.
The same thing can be done for MP, HP, HK links:
MP links = MP~LP (or LK, but obviously LP is easier to hit)
HP links = HP~MP (or MP or LK or LP, but obviously MP is easier to hit)
HK links = HK~MK (see above)
(Remember: you have to press both buttons almost simultaneously, but with the stronger-strength button a split-second earlier than the weaker-strength one)
It also works for cr.LK, BUT ONLY IF your character's cr.LK is not chain-cancellable.
cr.LK links = (while crouching) LK~LP
This method will not work with cr.LP, due to the fact that down+cr.LP+cr.LK is always interpreted as cr.LK. It will also not work with standing LP or LK because then it becomes a kara throw.
In summary, for p-linking to work, you have to understand two things:
When you do a slide input (press the second button one frame after the first button) like AA~BB, the first input (AA) will be repeated in the second frame (AA+BB).
Whenever the game detects an AA+BB simultaneous input, it will always give you the strongest attack button in the following order:
LP (weakest), LK, MP, MK, HP, HK (strongest).
Therefore HK+HP is executed as HK.
If you were to enter HK~HP, the game would read it as HK (one frame later) HP+HK, and execute it as HK (one frame later) HK.
However, if you were to enter HP~HK, the game would read it as HP (one frame later) HP+HK, and execute it as HP (one frame later) HK.
This is important to note if you want to p-link specials, like Ryu's forward+MP -> Shoryuken.
You can't p-link SRK with two punch buttons because you'd get the EX. So you'd have to p-link it with either MP~LK, or HP~MK.
(Disclaimer: not sure if you can p-link specials. At the very least, I couldn't pull off Ryu's forward+MP -> SRK)
Disclaimer: this linking method will not automatically make you a link combo god - you still have to get the timing right and everything. Like double-tapping, this essentially turns a one-frame link into a two-frame link by giving you two chances to nail the input window.
If you haven't figured it out yet, here's why it works:
A couple of days ago I was practising my Kara throws in training mode with Akuma (HP~LP+LK) when I noticed something - the game recognises my kara throw input as:
(second frame) LP+LK+HP
(first frame) HP
Note: The first input repeats itself when you do HP~LP+LK
And as we all know, whenever two or more attack buttons of different strengths are pressed, the game will give you the strongest strength attack i.e. if you press LP+MP+HP, you'll see that the game executes that as a s.HP.
In other words, if you were crouching, essentially the above input could be interpreted by the game as:
(2nd frame) down+HP+LP+LK (you're stuck in animation, but if you were not, this would have been executed as cr.HP)
(1st frame) down+HP (executed as a cr.HP)
So what happens when you're in the recovery frames of some other move, like, say, Akuma's cr.LP on Counter Hit (+5 on hit, and an additional +1 for Counter Hit)? You now have two chances to nail the one-frame window required for cr.LP to cr.HP link (Akuma's cr.HP has 6-frame startup), depending on if you hit the down+HP squarely on the window, or one frame before that. One-frame links become two-frames. Two-frame links become three, etc, etc.
This is effectively a double-tap HP. Only thing is, you don't have to cram both of your fingers on a single button, which surely takes some level of finesse to pull off, especially under pressure. It should make link combos, in general, a lot easier. Personally I can hit cr.LP to cr.MK about 90% of the time.
Some other examples you can try:
Ryu's cr.MP, cr.MP, cr.HK
Sagat's cr.LK x3 xx Tiger Knee
But of course, if you're very good at double-tapping then you may find double-tapping to be better/easier.
EDIT: rewrote a few things. Special thanks to everyone who's helped clarify things.