Boiling removes air from the Wort and the Yeast needs air to Survive and Work. Air, or better yet, clean oxygen needs to be replaced.
The ideal way is to inject clean, hospital type oxygen into the wort as it drains from the kettle into the fermenter.
This can best be accomplished by inserting a "T" fitting in the hose system in-line between the Kettle and the Fermenter. If you use in in-line cooling system or pump, please the "T" last in the line before the Fermenter.
The "T" needs about Five Feet of Tubing before the Fermenter to assure a good mis of Air and Wort. An Oxygen bottle and regulator can rented or purchased. Set the Oxygen Flow Regulator to approximately 2 pounds.
No need to rock or shake the fermenter as some recommend or having to place an aeration stone in the tank.