The session will sit not recording anything until the trigger condition is met, at which time data acquisition will start. That will trigger the DataAvailable event callback.
I do not know what rate you will be able to run at; in part it is going to depend upon how close together you might get triggered.
I seem to remember that people have a lot of trouble getting more than 3 or 4 kHz from the NI USB devices, but thinking back that have to do with transfer rates. If the NI device itself does not send anything until the trigger level is reached you might be able to set the sample rate high if you do not spend much computation time reacting to each of the triggers, but if the information has to be sent to MATLAB to evaluate whether a trigger occurred then you might not do nearly as well.
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