Old school bodybuilders preached about a “mind-muscle connection”, which in the past few years has been ridiculed by many in the fitness industry. This process involves actively focusing on the working muscle groups or as Arnie put it "you have to put your mind inside the muscle"
Interestingly, new research conducted by Schoenfeld and colleagues (2018), has shown that internal cuing offers some hypertrophic advantages when compared to internal cuing. After 8 weeks of bicep curls & leg extensions, the researchers found that the mind-muscle connection group gained more elbow flexor size but had similar quadriceps muscle gain (which they speculated was due to difficulty in focussing on the lower body)
Marchant et al. (2017), measured the EMG activity of the quads in isometric knee extensions. They again compared external and internal cuing. External cuing involved telling the subjects to push against the pad using their shin, whilst internal cuing involved telling the subjects the location of the muscles and to focus solely on using them to lift the weight. This sounds ridiculously simple, but the results were surprising. EMG activity was on average 20% higher in the internal than the external cuing group.
Snyder (2012), looked at the effects of internal cuing on the bench press. He looked at both high and low intensity exercise. At 50% 1RM, internal cuing increased EMG activity by 22% and at 80% 1RM, it increased EMG by 12%. He concluded that “verbal technique instruction is effective in shifting muscle activity during a basic lift, but it may be less effective at higher intensities"
To conclude, a practical approach may be to actively focus on the target muscles in the moderate to high rep ranges as it loses efficacy as intensity increases