Trainers get too caught up in the actual movement and the perfect technique. What this does is take out the actual brain and muscle connection. There is a big difference between lifting the weight up from start to finish and letting your muscle actually drive the weight up by using the optimal bio-mechanical line and contracting it intentionally at the same time, thus, utilizing the mind-muscle connection.
If you do a bicep curl, the keyword here is ''bicep'' not core curl, or knee slightly bent curl, or tight buttocks while you contract your abs and pulling your shoulders back and looking up at the ceiling curls. If you have a hard time with any exercises, the problem lies in who showed you. What I usually do is show the correct technique for a few reps and explain the major cues. Take the deadlift as an example;
1. Look forward
2. Lower back flat and butt down.
3. Lift the weight up.
Most trainers will say ‘’that’s it?’’. Telling someone who never executed this type of lift to keep the shoulders back and stick their chest out, while focusing on contracting their core while having their lower back flat as possible to pull (not lift) the weight up will only make it way more difficult than it should. Sometimes, I show it, and I tell them to pick up the bar like I did. Than I correct the main issues, one or two cues per set.
If your trainer wasn't able to show you in two sets or less than four cues, you might need a new trainer or the exercise might be too hard for you.