like MTI said, those marks are for downshifting.

for upshifting, you have the tachometer to refer to. But it is more difficult on downshifting to determine ahead of time, using the tachometer alone, if a downshift at a given speed/gear to a lower gear will over-rev the engine at that lower gear.

For downshifting, you look at the speedometer reading and the marking on the dial corresponding to your target gear - the speedometer needle should be below that hash mark for a safe gearchange.