A slide whistle has a changing pitch because the air cavity changes smoothly.
For the ocarina, you can do this effect by sliding your finger slowly off one of the holes.
About this exercise#
A slide whistle changes pitch because its air cavity grows and shrinks smoothly, with no fixed steps. An ocarina cannot move a slider, but it can fake the same glide. Instead of lifting a finger cleanly, roll it slowly off the edge of a hole so the opening widens by degrees and the pitch bends up rather than jumping. The trick is control at the fingertip: too fast and you get an ordinary note change, too slow and the tone thins out. Practise the roll between two adjacent notes first, keeping your breath steady, until the pitch travels in one connected sweep.