When smoking from a bong, punching a salad is the act of finishing a huge, tightly-packed salad bowl in a single breath. Such a feat is considered challenging, and is rarely accomplished by first-time smokers or casual stoners.
-"We were smoking weed last night, and Sam packed a huge salad bowl."
-"Did he punch it?"
-"Nah, Sam doesn't have the lungs to be punching a salad, It took him 4 hits to finish it"