Ahh, those take a special talent
There are a series of square holes, one above each round wire entry hole. Into that square hole you put a small screwdriver of just the right size (the largest you can reasonably fit in the hole is best, as considerable force is required). Then it is a little bit of a guessing game
. I am guessing that those blocks require you to push straight in to open the clamp. Once you have it open you shove the wire in its hole and pull out the screwdriver.
The guessing comes in because some of them have you pry up/down rather than press inward, depends on who made the block. For any of them, the force required is substantial, you are trying to overcome a spring loaded clamp that applies the same force to the wire as a screw-down terminal in order to make a good connection.
I really like spring clamp terminals, but in this use case they are a pain, because of the geometry, once you get the screwdriver in place it is blocking access to the wire entry hole underneath. Some designers don't ever think about the poor field person who has to make the last connection. If your fingers are anywhere close to as fat as mine a small pair of needle-nose pliers can be a good friend.
This is not the same terminal, but gives you a reasonable idea of the technology: