Basil Jaak originally didn't plan on giving Trista Stan the chance to poison him.
But the little witch was full of tricks, and if she didn't poison now, she might very well do so later. Constant vigilance was just too tiring.
Therefore, when Basil Jaak saw the powder in the palm of the little witch—apparently not very toxic—he changed his mind and decided to let Trista Stan "succeed" instead. Perhaps after her success, she would let her guard down, and that's when Basil Jaak might have a chance to win.
Of course, by "succeed," it didn't mean letting the little witch really get away with it. Otherwise, if he really got poisoned, how would he stand a chance to fight?
After all, Trista Stan was just a girl under twenty and naturally couldn't discern the strategy in Basil Jaak's mind. Seeing that Basil Jaak didn't move away, when she got the chance to come close, she sprinkled the powder on Basil Jaak.
"Ah, how fragrant."