Seedling are fragile and they break easily. To reduce chances of damage while planting seedling, growbed should be flooded to make the hydroton more movable.
For flood and drain growbed using siphon, the siphon need to be stop from operation either by stopping water inflow or by removing the siphon enclosure bell (cover).
With growbed flooded it is easier to arrange the seedling and get its roots deep down.
Once the planting is completed, reinitiate flush by restoring the siphon to its operational state.