This is a technique that seems to work for tuning across the bridge on this particular instrument. I have a few instruments, and I only do this on one of them. This instrument has three strings per note and is strung with piano wire. It's difficult to lift the strings up, or to adjust them along the outer rail, so this method helps me tune across the treble bridge.
I usually use a hammer that has no pair and a squishy leather side. If you use a hammer with hard leather or just wood, it can slide off the string and you wind up knocking your knuckle on the bridge, which kinda' hurts.