Quick succession. Due to the projectiles taking a while to arrive on target (especially for high-arcing trajectories) this gives you the effect of multiple projectiles arriving simultaneously on target: https://en.wikipedia.org/wiki/Artillery#Multiple_round_simul...

It also places some demands on fire control and loading to achieve the high fire rate; I'm not sure manually-loaded howitzers are capable of that in general (e.g. the German PzH 2000 automates everything from loading shells and propellant to firing).