I will say that, having read this paper rather quickly, I am not surprised by the conclusion, and neither should you. The paper is available for free at the link given above.
Abstract: Through in-class observations of teaching assistants (TAs) and students
in the lab sections of a large introductory physics course, we study
which TA behaviors can be used to predict student engagement and, in
turn, how this engagement relates to learning. For the TAs, we record
data to determine how they adhere to and deliver the lesson plan and how
they interact with students during the lab. For the students, we use
observations to record the level of student engagement and pretests and
post-tests of lab skills to measure learning. We find that the frequency
of TA–student interactions, especially those initiated by the TAs, is a
positive and significant predictor of student engagement.
Interestingly, the length of interactions is not significantly
correlated with student engagement. In addition, we find that student
engagement was a better predictor of post-test performance than pretest
scores. These results shed light on the manner in which students learn
how to conduct inquiry and suggest that, by proactively engaging
students, TAs may have a positive effect on student engagement, and
therefore learning, in the lab.
When I was a lab TA way back when, I tried to engage the students while they were performing the experiments. I tried to ask them on-the-spot questions, such as why do we need to measure the time for the pendulum to make 20 oscillations when all we care about is the time for one oscillation (period). I ask them many things about why they think we do this and that, rather than a seemingly-simpler and more direct measurements. I also ask them stuff related to the physics, such as during an experiment that used springs, what they think would be different if we were to do the same experiments on the moon instead.
Obviously, I couldn't do a study like these people did and investigate if what I was doing had any effects on the students and their lessons. However, I did get a lot of positive feedback from the course review. This new study reinforces the vital role that Lab TAs could play, and they need to read this paper to realize that they might have a non-trivial influence on the students.