It's really damn hard,pushing that H but to just for 1 frame,almost an impossible task if you don't have a different grip on your controller than you usually do.
Yeah it difficult . Slide your thumb in a quick motion from H to K you get the knee to come out but sometimes it does not. Doing combo challenge and doing 3 of them during combo I'm like WTF you serious lol.
It's easy for me and I play it on a Xbox One controller I just skim my finger on H+K (R1) while holding down K. All thanks to Sly Bass showing me how to do it.
If Sly Bass's method doesn't work for you, you can try my method, as xRELICXx posted above. I personally don't like having to switch controls just to play one character so I just use the default controls. It's really all about preference. His Guard break of Doom (Toryu Tenshin Hazankai) is much more difficult imo.
Like I said, its all about preference. If you found a method that works for you, then stick with it. Although even if you know the knee, there's a lot more to Akira than that. If you intend to learn him, be prepared to put in a lot of effort into him lol.
It's actually possibru to play him without the knee if you at least know how to do his 3H+KP466P+K, P+K43T6P (SPOD) and utilize his get-in moves properly (P, 2P, 46P, 66P, 666P, 6P+K, etc.) though you would miss out on more damage for a couple of his juggles.
Well good news I can now do it about 80% of the time now. Found a better way have my index finger on square and middle one on circle and just slide it across to right real fast