Yeast Sediment

In my opinion, you are better off taking your yeast from the secondary - it will be more free of trub.

Don't add it to left over wort - you'll wind up starting another brew.

Empty out your secondary into a sanitised container, and put the batch into the fridge.

On your next brew, let your yeast slurry warm up to room temp, and just pitch it into your wort.

It'll take off in a couple hours. Very impressive !

When you siphon your beer from primary to secondary, you are taking it off the trub, which is yeast, proteins, hops, and other vital nutrients to your yeast. While it will be free from the trub, that is the only benefit. Yeast from the primary contains cells that are still alive and have significant flavors you desire. There is more yeast and more active cells in the secodary.

One final point... the original message mentioned leftover wort from the primary fermenter. That is beer, not wort. Wort is unfermented beer, and even though beer in a primary for a week or two is not finished, that is still considered beer.

I reuse yeast all the time. From the primary! After I rack it off to the secondary I scoop the the nasty stuff off the bottom of the primary and and put it in a quart jar. Make sure you get some beer to cover the yeast. Cover with saran wrap tightly, and refrigerate until I make another batch. All I work with is sanitised. 1 quart can easily be used to start two 5 gal batches and I reuse yeast up to four times. I do not however reuse dry pack yeast.