About ZS

Sell your losers is in generally correct. However, the key word should be "Sell your losers quickly. Let your winners run".

But when it is too late to sell, you really should not sell your losers. Instead, analyze if the company is still a good company. If so, you should buy more instead of sell.

Use ZS as an example: I marked when it should be sold (cut lose). ZD just broke the down treand a few days ago. Although there will be lots of pressures on the way up, I believe the worst might be over for it. 

Disclaim: Not an advice to buy or sell. YMYD.