Legato

On a piano legato is a matter of finger overlap; on a string instrument it is a matter of bow distribution. Notes under a single slur share one bow stroke, so the player must plan how much bow each note gets, a long note near the tip that runs out of bow will die mid-phrase.

True legato also requires the left hand to cooperate: fingers should be placed before the previous one lifts, so the string never sounds an unfingered pitch in between. Where a passage crosses strings under a slur, the bow arm has to change level smoothly enough that the crossing is inaudible, which is why slurred string crossings are a standard étude subject.