Missable Achievement/Trophy Alert!: Overclocker
Complete Test Chamber 10 in 70 seconds
Move forwards until you reach the trench, look down and fire the other portal at the ground here and fall in. The momentum will carry you across over to the Companion Cube resting on the ledge. Pick it up and then fall back down through the portal below you.
Look up into the top right-hand corner of the room and on the white bit of wall here, fire a portal over here (it should be right across from where your Companion Cube is sitting). Now drop down and run over the jump pad again to be taken up to the upper floor.
Turn around and stand in the middle facing the jump pads (looking towards the exit). A Displacement Cube will come bouncing up towards you, so grab it, then drop down to the middle level below you.
Now simply drop back down to the ground floor (with your Companion Cube in tow), hit the jump pad and watch as you’re carried towards the exit. Set the Cube down on the large red button to open the exit behind you.