Many thanks to student Alex Blackwood who filmed them using the Sony FS-700 camera, filming at 800 FPS.
A very small amount of DRY magnesium and silver nitrate powders were mixed immediately before the demonstration. The burette was adjusted so that it released one drop of water every 5-10 seconds. Once this was achieved, the tile containing the magnesium/silver nitrate mixture was placed under the burette. The audience was warned not to look directly at the pile of powder but rather, to the side of it.
When the first drop of water falls into the mixture a vigorous reaction ensues which starts to spit and bubble. Also, some brown toxic nitrogen dioxide gas is released. Even before the second drop falls into the mixture, it suddenly bursts into a very bright white flash, leaving flames and smoke in the aftermath.
The reaction is: Mg(s) + 2 AgNO3(s) → Mg(NO3)2(s) + 2 Ag(s)
with some NO2 gas being released as well.
Magnesium is much higher in the reactivity series than silver and, therefore, displaces silver from the nitrate group. The REDOX reaction requires water to be initiated because the two powders are solids, and an intimate mixture is required for the Ag+ and NO3- ions to be able to move. However, this mixture is so sensitive that saliva from the speaker/demonstrator or even moisture in the air can set it off.