Undoubtedly, supporting reflection in student teachers during university-based training is one of the most sustained measures to attain teacher professionalism. Therefore, at the Freie Universität Berlin an on-campus seminar designed to relate theory to practice and vice versa -- the so-called "Teaching and Learning Lab (TLLS)" -- was implemented over the course of five terms to stimulate reflective skills of English and physics teacher trainees. Investigations on the effectiveness of three types of the "TLLS" (no video and two types of video-supported reflections) compared to a parallel group (PG) and a control group (CG) occurred in a Mixed Methods quasi-experimental study. Reflective skills were elicited with vignettes, relevant covariates with questionnaires. Reflective development was then traced in the dimensions depth and breadth employing a Qualitative Content Analysis. MANCOVA and regression analyses revealed a substantive increase of reflective depth for English and physics teacher trainees and breadth development for English "TLLS"-participants in contrast to both, a PG and a CG, even when controlling for the subjects' individual prerequisites.