The cable cross-over is a classic single-joint exercise for the pecs, but the way many lifters do it is more like a press. 

You can tell by watching the elbows: If they close and open up, similar to a bench press, the triceps are heavily recruited into the movement. That turns this exercise into a multijoint movement. 

That's not necessarily a problem...except that the cable cross-over is a movement meant to isolate the pecs! Nearly all lifters choose cables to isolate the pecs. If you want to do presses, you can use much more weight with dumbbells or a barbell on a flat bench. So why would you want to press with cables? The answer: you wouldn't. 

Make This Change: Lock a bend in your elbows halfway between 90 degrees and where your arms are perfectly straight, and hold it there throughout the movement. If you're not sure, watch your form in the mirror; you must maintain that same elbow position from start to finish. And go light to avoid turning your cross-over into a press!