Mark,
You can shift from low to high while underway, and with practice from high low. But you cannot shift into gear from a standstill with the engine running. When you come to a halt, you stop the engine. Usually by leaving it in gear and lifting the exhaust valve lifter or just shutting the throttle and letting the engine die. If the transmission is not already in low, you shift it into low. You arrange the throttle and air lever where experience tells you the engine will start, lift the exhaust valve lifter and paddle off. At a walking pace you can release the exhaust valve lifter, at which point the engine will (hopefully) start).
-Doug
