Backstrokers need to be on their side to pull effectively and thast...

They obviously can’t be on both sides at the same time, so they have to quickly shift sides to minimize the gap in propulsion between the two arm pulls.
The arm recoveries are critical for making it happen.
The momentum of the arm swinging over the top helps swimmers quickly switch from the right to left and vice versa.
As soon as one arm is done create propulsion the one arm can pick up right where it left off.
It allows swimmers to keep drag down and stroke rate up, all without sacrificing propulsion.
That adds up to more speed.
Here’s a simple set to improve this skill: