When you have the program launcher grid on screen you will see that the grid is 3 rows deep by 5 columns wide.

If you look at your keyboard the following keys correspond to the grid positions described above: QWERT, ASDFG, ZXCVB.

Pressing the relevant key will launch the application in the corresponding position, e.g. pressing Q will open Web, pressing B will open Settings (assuming you still have the default positions in place).

Of course, this will only work for the first 15 apps.