I place my hands on either side of the fermenter, and give it a sharp quarter turn, and back. I typically do this three times. This "swirls" the wort without sloshing it. There is no oxygen in the fermenter, it all has been pushed out of the airlock by CO2, so aeration is not an issue. The airlock may bubble a couple of times when you rouse the yeast, not a problem. There may be a slight increase in airlock activity in the hours following rousing.
Rousing the yeast like this simply re-suspends some of the "lazy," live yeast that has fallen out, from off the top of the trub. Any dead yeast cells suspended, will only act as nutrients for the living.
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