Interesting! I've never had success triggering them, though that's not surprising given I'm on an aluminum and carbon frame.
There was actually a kickstarter a few years ago to build a magnetic loop you could put on your bike to trigger the sensor via a button press, but unfortunately it didn't meet its funding target and seems to have gone underground: https://www.veloloop.com/faq/
If you've got metal rims, and the loops are reasonably tuned, you should be able to trigger them if you're well positioned. Apparently, adding a copper wire loop under your rim tape will work for carbon fiber rims as well.
There was actually a kickstarter a few years ago to build a magnetic loop you could put on your bike to trigger the sensor via a button press, but unfortunately it didn't meet its funding target and seems to have gone underground: https://www.veloloop.com/faq/