The idle state, which controls the pulse train polarity, is applied to the signal when a task transitions to the Committed state (which happens automatically when the task starts). The idle state polarity also determines the state of the initial delay.
When you set the idle state to low, the generation starts low for the initial delay, then transitions to high for the high time. The low time is ignored for the first pulse, but will be repeated before the high time for each subsequent pulse, as shown in the following illustration.
When you set the idle state to high, the generation starts high for the initial delay, then transitions to low for the low time. The high time is ignored for the first pulse, but is repeated after the low time for each subsequent pulse. In both cases, the output rests at the idle state after the pulse generation completes.