This is typical. There is no speed change made physically by the capt. But as the boat goes into the turn you lure or line starts to slowly drop and slightly slow giving it a different action. As the boat comes out of the turn your line starts to staighten out it the regains its momentum and picks back up to its original troll depth. Some days if fishing is slow try small S trolls to change things up instead of just straight trolls. Boat traffic permitting of course.

Lures on the inside will drop and slow a bit and the outside line will speed up a little. It doesn't hurt to change speed a little up or down to trigger a strike.