FINGERS AND KEYS

i didnt really understand the concept of playing more into the keys and feeling that the keys and the fingers are one than just taping on the keys when playing fast. i notice its a different sound and in one of the videos you talk about the fact that when playing at a high speed there isnt really a conncetion with each finger and the key (like when running fast we dont feel the contraction of the calf muscle),but is it ok to say that there is a bit of pressure when the fingers are more into the keys-when there is a skeleton connection with the keys? i also noticed that when the skeleton connection happens the fingers are less straight,so what is the physiological difference between straight or a bit curles fingers in relation to the skeleton connection with the keys at a high speed?