What quick action by Saknis plays an instrumental part in Matt's recovery in Sign of the Beaver?
I believe the action you are referring to is Saknis's treatment of Matt immediately after the bee attack. Matt, lying on the ground, was "too weak to struggle;" his skin "seemed to be on fire from head to toe, yet he could not stop shivering." Understanding that the bee stingers contained "poison" and needed to be removed, Saknis moved his hands gently over Matt's face and neck and body, "probing and rubbing at one tender spot after another." By removing the stingers immediately, Saknis minimized the amount of poison that went into Matt's system, enabling him to recover that much more quickly.
Saknis actually did a number of additional things to aid in Matt's recovery. First of all, he pulled Matt out of the water when Matt was thrashing about, his feet tangled in the weeds at the bottom of the pond. Had it not been for this action, Matt, in his panic, might have drowned, and there would have been no recovery to speak of. Saknis then carried the semiconscious boy back to his cabin and gave him some "bitter medicine" to help in his healing. He also brought Matt food until he was well enough to get his own, and, seeing his need, a"rough sort of crutch" for him to use until his injured ankle was stronger. Finally, when Saknis learned that Matt had lost one of his boots in his escapade with the bees, he brought a pair of moccasins, made by the women of his tribe, that were "handsome and new, of moosehide, dark and glistening with grease, tied with stout thongs that were long enough to wrap about his ankles" (Chapter 5 and 6).