The RCD will trip if there is 30mA or more difference between the current in the P and N conductors.
It is possible, if you are wearing rubber boots, in the example you give, that you could grab the two cut conductors in one hand and have no current flowing to earth.
That would give you a painful burn to the hand, and the RCD would not trip.
"Luckily" the current would not be flowing through the rest of your body and might not stop your heart, I don't know.
If you only grabbed the live conductor, not the neutral, and there was no current flowing to earth, you would not get a shock and the RCd would not trip. No problem
However, if you were standing on damp grass in bare feet, some of the current
would flow through your body to earth, and the RCD would trip.