Grimrock

After defeating the cube. I'm in a room with a pressure plate, which activates a energy-bolt, which in turn opens up another corner of the room. But I can't see how to proceed from there.

the plate opens the portcullis

already forgot the energy bolt and the other .. noise?

If you mean I have to block the bolt, I don't see how. I step on the plate (or drop a rock on it) and the bolt hits the receptor before I can even take a step.

You just stand in place to block it and then toss something to the plate. If it's still blocked, find the secret button to open the wall.

Complete explanation
The pressure plate does three things when activated:
- it raises the portculli
- it fires the energie bolt
- it raises walls beside the demon head. To keep this walls open the pressure plate has to stay activated.

When the energy ball hits the receptor the portculli closes again so you need to block the energy bolt from reaching it.
(-> recall "Beasts and Menagerie" entrance)
This also means that you need to throw something onto the pressure plate while standing in front of the receptor.
(-> recall 1st secret in "Times and Tide"; self-teleportation)
Now before you can toss anything onto the pressure plate you need to raise a wall between it and the receptor. This is done with a secret button which is revealed upon activating the pressure plate.

Step-by-Step
- hold the pressure plate down with an item and wait for the walls to rise
- push the secret button behind the raised wall to raise another wall
- pick up the item from the pressure plate
- stand in front of the receptor, blocking it this way
- throw anything onto the pressure plate