What you are training
A metaphor is a comparison the listener completes. "Love is a battlefield" works because the listener supplies the wounds, the sides, the exhaustion. A weak metaphor supplies nothing, either because the two things are too alike or because the comparison has been made so often it has stopped being one. Building good metaphors is a process: pick the target, list its qualities, find a source that shares two or three of them in a surprising way, then test the comparison against the whole song so it does not collapse in the second verse. This drill walks the process with a timer: a target idea, a brainstorm of sources, a chosen comparison, and a line that uses it. The reveal scores freshness and fit, and flags the comparisons everybody has already made. It is the drill behind the extended metaphors that hold a whole chorus together.
After this drill, the plan: The daily plan follows this with an image-system audit on one of your songs, so a metaphor you build stays consistent from first verse to last.