Conservation of angular momentum for a system on which
no torque acts is the physical principle behind the gyroscope.
A rotating object is suspended in a manner that prevents there from
being any net torque acting on it. Under these conditions we have
In other words the axis of rotation will point in a fixed direction and the angular velocity will not change. I have a gyroscope which can be handed round and you will see that it always points in the same direction no matter how you turn the outside assembly because by doing so you are not imparting any torque and hence you cannot change 
. Interestingly by the way the gyroscope does
not point to a fixed direction on earth it points to a fixed direction, period. That is as the earth rotates the gyroscope will remain locked
to its initial direction and hence to an earth-bound observer it will appear that is rotating when in fact it is the earth that is
rotating and the gyroscope that points in a fixed direction. In fact you should even be able to see the direction pointed to at a specific time of day change through the year as the earth moves in its trajectory around the sun.