You can piano to help provide more button inputs (such as hitting the small reversal window for a reversal DP) but you can’t use that for “mash” specials.
I thought I was pianoing in HDR, but apparently I was just pressing one of the buttons 3 times when I was pianoing or something 
pianoing in ST is important but not for getting moves like HHS/Electricty,Chun Li kicks.
You do pianing on certain moves to get more opportunity to have the correct timing on your moves. Unlike SF4, your inputs have to be pretty precise in ST/HDR.
For example, you want to do a wake up DP on a jumping opponent. You can piano the reversal like this: F,D,DF +Jab, **(Strong,Fierce) **
Piano the Strong, and Fierce after the Jab input. You get 3 chances to be get one of the punches correctly. This can be done with anything really but is most useful when needing 0 frame reversals or making sure to get the timing right on SPDs, or SPD supers.
my 2 cents
Actually due to negative edge, you get SIX chances to hit a special using that method, but your piano input has to be super clean.
well yeah, but like, was trying to help the OP get a “basic” understanding of pianoing in ST. But your right, technically you can get more then 3 chance to get your move out with negative edge properties.
correct me if im wrong, but pianoing is basic in SFIV in order to perform some advanced combos wigh Gen and Honda, isnt it?
it is used in this way, but isnt technically required. You can actually press 1 button 5 times to get it out. Technically.
Well the question was about ST, but yes…in SF4 Gen, Honda, and Chun all have combos/links that you can really only do with careful piano inputs. I mean…if you have crazy mashing skills, you MAY be able to hit some of these (NOT the gen one) but its too hard and it can be inconsistent.
Example: Honda jab HHS - jab~strong~fierce~jab~fierce
Example: Gen st forward into hands - jab+forward~strong~fierce~jab~fierce
Example: Chunners c.short x2 into ex legs: short, roundhouse~forward~short, short+forward, short+forward
There are actually numerous ways to do the chun combo above, but this method is the one I prefer.
thanks parabellum, really useful info there 
To make this explicitly clear:
SF2 (and SF3 and the SFA’s) read “mash moves” by individual buttons. If you’re trying to get the strong version of Honda’s Hundred Hand Slap, pressing jab and fierce don’t help your cause. At all. It’s looking for X number of presses on one particular button within the required time window.
SF4 (and CvS2 if you care) look for presses across all punches or all kicks, where the fifth press (in the required time window) determines which strength of the move comes out.
Like The Mullah
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In street fighter HD Remix, mash moves only require 3 inputs in a given amount time(dont know exactly how many frames) but in ST electricity requires 5 and HHS requires 4 hopes this helps
Nobody uses fierce mash moves in Sf2 because they are ungodly difficult to pull off without buffering off a jump-in. Medium is a little easier since it requires one less input. That’s what you see guys like Kusumondo (best Japanese Honda) using.
Thanks for the help peeps, I already knew about pianoing for revrsals though. Funnily enough, I used to mash one button for mash moves in ST when I used to play it on GGPO, but after I played SF4 and CvS2 I learnt how to do lp xx elec with Blanka and tried doing that when I went back to GGPO recently, not realising it wasn’t possible 
Assuming you have a stick, double tap the appropriate button by quickly alternating taps between your index and middle fingers. This is enough to ‘hide’ fierce HHS in a cr. mk, for instance. But good luck getting cr. lk xx HHS in ST!