Gyser works on the principal of heating of water
when gyser system is full of water the presure of water in bottom of gyser is more than the top
the greater at the presure of the water make the water boiling even if it reacehes temp. upto 150 c
if the presure at the top of the  water colum of the water is reduce in some way it make the water to force the water ot of its hole