...I don't think it shortcuts that much of the trail, but the big benefit is avoiding the first North Fork crossing.
It is the old stock trail...
It is not off-limits.
It knocks off 1/4 mile or about 5 minutes (downhill). 27 switchbacks I do believe.
Going up, it can be "brutal" for some unlike the smooth gradual on the MWT, but yes you do bypass that first major water crossing. I advise on the descent to take your socks and boots of and soak your feet after a long day of hiking! Trust me, it does do your feet wonders....one word...exhilirating whether it is freezing cold or not.
Coming down it can be brutal due to fatigue and smelling that food from the Portal kitchen. It ends at that "big rock" you see at the end of the loop by the day use parking area. If you cannot see that rock, you need beta carotene and a beer.
There are other "trails." You can see one trail below just before the first "hairpinish switchback" going up at the start on the MWT. You just have to scan the terrain very closely and you will see it. If you blink, oh well.
Another "shortcut" is heading up, and I mean up, alongside Whitney Portal Falls. Not recommended unless you are a mountain goat. Sensitive habitat there. One slip could mean a nasty fall and someone will be fishing you out of Lone Pine Creek.