If the recipe calls for small pieces, these will typically have been sliced 'across the grain', i.e., across the direction of the muscle fibres, and marinaded.
i marinade my chicken in milk for an hour first, rinse the milk off , then poach it in a pan of boiling water rather than fry it - works but dont ask me how!
frying in oil will seal the chicken on the outside but will evaporate a lot more moisture than poaching, which will release the fats and should as it is in water, stay more moist,
the more water and fats that are lost in the cooking will make the meat dry and tougherless tender,
i think they probably cut the meat and poach it/blanch it quite a bit, then stir fry for the minimal amount of time to bring it to a fully cooked state..
tired and hammered, so this might not make much sense sorry!
I know that high class chinese restaurants pound good quality chicken to tenderise it, then coat in egg or cornflour before frying it. Chicken you get from cheaper chinese restaurants are sourced from frozen chicken with high water content injected into the meat. This gives it a very tender texture.
I think best way to get a naturally tender chicken breast is to poach it, then transfer chicken to a bowl of ice cold water to cool down. The chicken turns out succulent and tender.