[Michael R 1974]

But with such 'underexposure' of the negative the highlights usually benefit through better separation, as they are not so far up the exposure curve and into the more contracted 'shoulder' area. - David Lyga

Depending on the film/developer combination, the highlights should maintain full separations up to pretty high levels (at least ~Zone XII in ZS parlance) with good-to-useable separations thereafter up to ~Zone XV, provided normal or mildly contracted development is applied. Big time underdevelopment and/or compensating procedures significantly flatten highlight contrast (effectively blocking highlights but with lower densities) so they must be carefully used. This is why when dealing with very high contrast subjects I suggest considering printing techniques when making the exposure/development decision at the scene.