I'm not certain what you are using, but I know root trainers simply as trays with very narrow, deep cells that allow the seedlings' tap roots to develop fully. No propping, opening, or spilling involved.

Rootrainers come in 2 sizes, i use the longer ones for sweet peas, the smaller ones for just about anything else you want.
Both sizes fit in the same size outer container, holding 8 inserts of 4. You fill the container with the 8 inserts, once full the whole thing is rigid. Then you add the compost.
I would recommend sowing seeds in a tray, then transplanting into the rootrainer.
As to opening, be guided by the plant size and have a look underneath the rootrainer. If you can see nice roots coming out the bottom you won't be far off.

Aha! I found the brand name product, finally. It looks more elaborate than I'm used to, but potentially less traumatic to the plants when planting out. What I am familiar with was produced for the forestry industry, to grow tree seedlings, which have stronger "handles" than the average pea or bean!