It's actually a pretty generic plan.
"You guys should spin off this one profitable division, and then, like, FOCUS MORE on the other parts"
isn't much of a plan.
They probably want a pretty radical restructuring that dumps everything that's not looking to make a profit in the next quarter or two, and divides the thing up into less of a monolithic thing and more of a bunch of separate pieces. This pretty much always makes the stock go up. Whether it obliterates or saves the company itself in the end depends on a lot of stuff, and usually isn't of all that much interest to these hedge fund guys. The way hedge funds work, they're really not set up to care much about long term health of the target companies.
The stock went up, basically, because the market sees that Third Point is likely to force Sony to do some stuff that will.. make the stock go up.