At a coaching this week with Julian Reed of Arizona Opera (which was an excellent coaching btw), I was instructed to sing this exercise. The goal is to take the pure [i] and [e] as high as your upper passaggio (flip, bridge, break). Use the front of your tongue to pronounce, but keep plenty of space in your throat to keep the depth. One image is to use an “uh” or [a] in the throat while pronouncing the other vowels.