Setting the Trigger

The Trigger allow you to see the signal stable on the screen since the Trigger set what is the starting point for drawing the signal. Even if the input signal can be periodic in some way, on each scan its position can be found translated since the acquisition time is random and it cannot be synchronous with the period of the input signal.
The starting point for a signal can be identified with:

a Voltage Level (Trigger Level);
a tendency (or Slope) which means if the signal is passing the Voltage Level going up or down.

Once decided the kind of trigger (AUTO, NORMAL, SINGLE), you can set the SLOPE (arrow up or down) and the input Channel (only CH1 and CH2 are working and they are oscilloscope depending).

AUTO Trigger: set the Trigger Level as the middle level of the input voltage.
NORMAL Trigger: allow you to set the Voltage Level for the trigger. Every time the condition is met the signal is visualized on the screen.
SINGLE Trigger: as Normal Trigger but will show the signal just at the first occurrence of the condition, then it stops the acquisition. You can resume the acquisition by pressing the play button.

More about Triggers

There are many known ways to "Trigger a Signal" but currently HScope allow to use just the most simple kinds.

Limitations

This trigger can be implemented at software level or at hardware level (check the information about the oscilloscope to understand which kind is supported). In case of Software Trigger the Single Trigger mode cannot work at high frequencies since it depends from the acquired data which is just a little part of the total signal data at high frequencies (not all samples can be transferred via USB to the software). Since we can acquire just few parts of the signal, when a spike happen during a reading, it is detected from the software and the Single Trigger can show it. If the spike happen when the device is not acquiring data, then nor the hardware nor the App can detect it.

Acquiring 1024 sample at 200KS/s means to read a total period of about 5ms. If we have 25 scans of the signal each second it means we can see a maximum of 125ms on a period 1s (we can see just the 12.5% of the signal).
Increasing the Rate we can see even less since the acquired period is shorter. 
Cypress FX-2 based devices allow a correct Single Trigger with HScope up to 100KSa/s.