Trigger delay allows a captured signal to include some data before or after the trigger event. This is done by defining some number of points, or the percentage of the total Block Size, that the capture occurs after the trigger event. For example, if the Block Size is set to 1024 and the trigger delay is 10%, the data capture will happen 102 points after the trigger event.
A negative trigger delay is more common for transient data capture. Negative trigger delay means that the data capture will include data points before the trigger event. For example, a -10% trigger delay means that the data capture will include 102 data points before the trigger event with Block Size 1024. Some instruments call a negative trigger delay a Pre-Trigger. The following picture shows the concept of a negative trigger delay: