“sinks down.” If you set fire to a pile of dry grass, as it burns up the grass on top sinks down into the flame until it is on fire and then consumed. This verse gives different illustrations of the destruction of the wicked, who will be consumed in the Lake of Fire until they exist no more. They will be like stubble or grass that the fire burns up, like a root that rots away, and like a blossom that turns to dust and is gone.
[For more on annihilation in the Lake of Fire, see Appendix 5, “Annihilation in the Lake of Fire.”]