Refer to figures.
PRINCIPLE OF THE AIRSPEED INDICATOR (ASI)
When an aeroplane is stationary on the ground it is subject to normal atmospheric or static pressure, which acts equally on all parts of the aeroplane structure.

In flight, the aeroplane experiences an additional pressure due to the aeroplane's motion through the air, which is known as dynamic pressure, and is dependent upon the forward motion of the aeroplane and the density of the air.

The airspeed indicator thus measures the pressure differential between the two sources, and provides a display indication graduated in units of speed.

BLOCKAGES - PITOT TUBE (figure 1)
A blockage of the pitot tube, possibly due to ice, causes the ASI to not respond to changes of speed in level flight.

The capsule behaves as a barometer or altimeter capsule, and reacts to changes in the static pressure. If the aeroplane climbs, the ASI indicates an increase in airspeed (over-read) and if it descends, it indicates a decrease in airspeed (under-read).

BLOCKAGES - PITOT TUBE (figure 2)
A blocked static source during descent will mean that the ‘old’ (higher altitude) static pressure surrounding the capsule will be lower than it should be, so that if the pitot supply is normal the ASI will over-read.

A climb with blocked static source and normal pitot air will result in the ASI under-reading.
With a blocked static source, the ASI over-reads in a descent.

A useful mnemonic for examinations is PUDSOD, which stands for:
‘Pitot Blocked: – Under-reads in Descent Static Blocked: – Over-reads in Descent’

The opposite of this can also be concluded:
‘Pitot Blocked: – Over-reads in Climb Static Blocked: – Under-reads in Climb’