I'm not certain, but I think you'll be left with squared off blades, which will probably brown at the cut ends first, and then die off. New shoots may come up from the crown.

You'd be better off dividing it in the spring, and leaving a smaller plant. If you don't have any place to put the new divisions, I can help you out. ;>)

Ornamental grasses grow the same as your lawn. Not from the tip but from the bottom.

-- If you cut your porcupine grass to the ground mid season, it will regrow from the base. This sort of cutting back will delay blooming and, in coldish climates, the plant likely won't have sufficient time to bloom.

-- If you cut it just half way back, most of the existing stems will continue to extend upward and more shoots will develop at the base. Again, likely to delay bloom.

If you need a shorter plant for that site, best to choose something else.

No matter how early or late you cut this grass while it's in growth its foliage will retain its squared edges all season; the plant's natural form will be compromised; and seasonal growth will not disguise your pruning act, prompting others to question your gardening ability and thinking. Per Donn's or Jean's advice, either divide every year (and deny the grass from ever achieving its potential) or move the plant and replace it with one whose characteristics fit the site.