Use the File Trigger feature in the sampler's Advanced tab.
The File trigger feature allows external control of a sampler. Netprobe will monitor all configured files for a change in modification time. When a change is detected, the sampler will sample. This setting is typically used when regular sampling has been disabled.
Whenever the control.txt file is updated (i.e. appended with a new line), the sampler will do a sample. You can also control the number of times that a file trigger can be triggered per minute. For this example, the sampler will sample 3 times within 1 minute if the file has been updated 3 or more times.