questions
But of course he didn’t get the chance, so he simply raised his hand in a little no-offense-taken salute. The truck’s taillights flickered, as if she were reconsidering. Then they went out and the truck sped up.

Question 1: In this case, 'in' is used for talking about the way he raised his hand, right?

Question 2: The truck’s taillights flickered ( only once )? Or The truck’s taillights was flickering?

Question 3: Does 'The truck’s taillights flickered' imply that the car almost stopped or the car rolled slowly?

Question

questions
But of course he didn’t get the chance, so he simply raised his hand in a little no-offense-taken salute. The truck’s taillights flickered, as if she were reconsidering. Then they went out and the truck sped up.

Question 1: In this case, 'in' is used for talking about the way he raised his hand, right?

Question 2: The truck’s taillights flickered ( only once )? Or The truck’s taillights was flickering?

Question 3: Does 'The truck’s taillights flickered' imply that the car almost stopped or the car rolled slowly?

Answer

1. Yes

2. Yes, probably, though the amount of flickering is not so much the point,. What's important is the meaning of the flicker. If the duration were important, the writer would probably have said "the taillights were flickering"

3. The meaning of the flicker is that the person  was driving away, or preparing to drive away, and then tapped the brakes because he or she considered stopping (i.e changing his/her mind about driving away). Why the person was driving away is probably significant, and the 'flicker' shows he or she is uncertain about the decision for some reason.