The slider sets the total volley damage, split across the projectiles. Fewer projectiles = each one fat, strong, big and slow; more projectiles = each thin, weak, small and fast — the total stays roughly the same. Projectile count is the choice between concentrate and spread. More power and more projectiles = a longer charge.
◀ Space · narrow beamTime · wide burst ▶
Same number of projectiles, delivered differently. Right (time) = a wide burst that leaves almost together. Left (space) = a narrow beam strung out one after another over time. You can't concentrate in both space and time at once.