One of the most important timing elements in backstroke is the...

Right when the hand hits the water, swimmers need to rotate to that side to create the necessary space to set up the stroke.
Or both.
Backstroke is a lot more effective when swimmers can get their entry timing on point.
A big kick on a kickboard is helpful, but the real goal is creating speed through the kick while swimming full stroke.
Here is one way to put real demand on the legs while still swimming butterfly.