You control the direction of your trolley either with the keyboard or the mouse, you can tell Piston which you want to use in the Preferences box. When you first get Piston, the keys will be set to: Right arrow-move right, Left arrow-move left, Up arrow-stop, 'F' - stack block, and 'C'- change speed.
The trolley can go slow,medium, or fast, you can change speeds either by clicking in the S M F boxes or by using the speed change key.
You stack a block by moving under the position you want the block, and clicking the mouse or pressing the Fire key.
You can reject a block (if it's too weak for your purposes) by clicking the mouse or pressing the Fire key while the trolley is under the new block chute. The block will slide offscreen and the next will drop onto the trolley.
