Current HL pedagogy recommends focus on form approaches be used in HL classrooms (Beaudrie, Ducar, & Potowski, 2014) but within this broad category many different techniques can be used. To date there are only a few studies that have compared HL learning gains on explicit and implicit focus on form instruction (e.g., Beaudrie & Holmes, in press; Fernández Cuenca & Bowles, in press), and these have found explicit instruction to be more beneficial. If we are to understand how to maximize instructed HL acquisition, it is essential to understand why this is the case, and to do so, we need more evidence, beyond just pre-posttest gains. This study compares two groups of Spanish university-level HL learners who completed either an explicit (Processing Instruction, PI) or an implicit (Structured Input, SI) computerized instructional module on the use of indicative and subjunctive in adverbial clauses of time. Results rely not only on behavioral measures (pre- and posttest scores) but also on introspective data collected either through think-alouds while learners completed the instruction or post-instruction interviews. Although some learners made large improvements in the implicit condition, there was far more individual variation in this group than in the explicit group, which made more consistent and lasting gains. The introspective data provide information to contextualize the learning gains seen in this and other studies and show that explicit techniques, including rule presentation, can serve to orient learners to attend to the target form in ways that implicit techniques often do not.