Tapping into space while executing the action will act like the "Shift" key. For example, if you are sketching a line, while you are dragging the line with your finger/pencil along an axis (green, red or purple inference line) and you tap in space with another finger, you will "Shift Lock" onto that axis. You can tap again in space with an extra finger to turn the "Shift Lock" off again.
Hope this helps with your modeling!
-Heather
