In collaboration, group knowledge awareness (i.e. being informed about the partners' knowledge) is used to effectively communicate and to efficiently coordinate interaction. Computer-mediated collaboration impairs the establishment of group knowledge awareness. Technological support for group knowledge awareness can compensate for this shortcoming and is realized in a group knowledge awareness tool that visualizes the collaborators' self-assessed knowledge. In an experimental study, we varied the availability of the group knowledge awareness tool and investigated the mechanisms of collaborative learning with and without the tool by adopting a contrasting cases approach. Comparing dyads selected for their notably low or high learning outcome in both experimental conditions revealed distinct learning mechanisms with and without the tool: more individual elaboration was found in high compared to low outcome dyads in the control condition, while more collaborative elaboration was found in high compared to low outcome dyads in the group knowledge awareness condition. Using the tool for coordination in dyads with large knowledge differences, that is distributing activities according to the knowledge difference, set high outcome dyads apart from low outcome dyads, when they were provided with the tool. Implications for the design and practical use of group knowledge awareness tools are discussed.