The dashed lines drawn on Ei in thew figure locate Vut and Vlt.
At time t=0, Ei is below Vlt, so Vo is at +Vsat. When Ei goes above Vut, at times (a) and (c), Vo switches quickly to -Vsat. 
When Ei again goes below Vlt, at times (b) and (d), Vo switches quickly to +Vsat. 
Observe how positive feedback has eliminated the false crossings  