If you put /attribute @p minecraft:generic.max_health base set 1 into a repeating command block that is always active. Then, type /kill in chat to kill yourself. Next, you want to respawn. This will make the player's health bar show half a heart. Then, fall off of a cliff that is at least 10 blocks high. If done correctly, the play will take fall damage but won't die even at half a heart.