If you have a square wave with 50% duty cycle, centred about the 0V, you will get a triangle upwards when you have a positive pulse and a triangle down when you have a negative pulse. Repeat multiple cycles across the screen and you will see a sawtooth wave, the more cycles on the screen then the more sawtooths. It is not related to the time as it is just looking at the screen trace. Now if you change the duty cycle the saw tooth will move up or down, the greater the change, the greater the movement.
Please note that with streaming trace this will only appear once the screen is complete, the maths channels currently require a screen of data before they can be drawn (watch this space for a future update on this