The Dangers Of Locking Out Your Elbows
 

So why should you avoid locking out your elbows? For this question let us use a bench press example. During the top of the bench press you opt to momentarily lock out your elbows whilst lifting 100kg. At this exact moment the entire stress of the exercise is transferred to your elbow joints rather than the target muscles. Not only does this reduce the exercise intensity, as mentioned above, by providing a micro break during exercise, but the joints are placed under immense strain that can easily result in damage.

Full article here: The Dangers Of Locking Out Your Elbows | FitnessBeans.com

i never really do lock them out just under a lockout :D