Although quiet light weight and looking much like a simple tube launcher - they were sophisticated & adjustable for range and could be very accurate - having a rifled barrel.
What they were NOT was .... NOT FIRED BRACED ON THE KNEE OR LEG !
Trigger fired at an angle of 45 degrees - the range was adjustable using a scale and the grenades are an efficient - clever design .. for example they slide freely into the barrel when loading - but have a driving band that expands and engages the rifling when fired.
As is often the case, FORGOTTEN WEAPONS.com explains this system better than I might: Link to their video:
TYPE 89 Mortar Shell - Grenade
-Brass Driving Band at Base.