you push the edge of the tappet, to the side of the cam lobe.
also there not constantly compressed between the valve sping an the cam lobe there is free play in there. pokeing them with stick effectivly adds some pre load to the tappet so when you press down on the tappet with lots of free play it makes the noise change. on an 8v i probly woldn't bother tho id just swap the lot for a known good set thats hanging around or a new set, but when you get 16, 20 , 24, 30, 32 and 40 v enignes with a ticky tappet it gets more and more usefull to be abel to track down the offending one