when activateing a piston that is connected to a block tower by for example if it is activated by a lever you break the piston you move it down 1 block ajnd break the lever it stay activated without a source this can also be relpicated if you have two pistons stacked on eachother
if you activate the top piston with a redstone block it will turn on both pistons makeing them extend if you break the top iston and then break the redstone block it will stay activated without a redstone source also any block update to it for example placeing a block next to it will cause it to reset