HBP/PE18

I've been working on HBP/PE18 for some time now.  I am having the following issue.  As I move towards the 4th set of 5 reps, my lower back starts to arch.  I know this is incorrect.  I've moved back to lower reps and worked my way back up but continue to have the same issue.  What should I do to help overcome with issue?  

The video below shows the 4th set.  Normally I slide my feet on the wall but I was without socks.  I think this still show the issue but if not I am happy to shoot it again.

You could move closer to the wall or turn around and do them stomach to wall. Either way you will need to pull the ribs firmly down and protract the shoulders to clean the line up  at the moment aa you can see you're compensating with your body from the stomach down to reach the wall. You may find as a consequence that the shoulders are required to work harder due to less assistance from the pecs; power through, hspu are worth it