Using a gradual string-crossing action allows for a more sustained sound.
Slow down the action and watch the bow move gradually and continuously to the next string. As you approach the new string, you can be moving towards the edge of the old string to make the string change motion less abrupt.
Make sure the left finger on the new string is ready in time!
SEE
Variation 14 – Round bow, arpeggios with double stops
Whole-arm string crossing