"Balance the hand to suit the task. This hand position suits the 4th finger with a high 3rd finger. This (next) hand position suits the 1st finger and low 2nd finger. In between we have a hand position that we use most of the time."
Here Fintan demonstrates a hand position centred on 3rd and 4th fingers, then one centred on 1st and 2nd fingers.
Then he demonstrates the most common hand position where all fingers can be reached with relative ease at moderate or fast tempos.
If the passage is very slow or the player is using extensive vibrato then the hand position may change to suit each individual finger in turn....