Question - why so much tension on downrigger rods?

[Jeffonc]

I see pics of the striper and salty boats with all the rods out, and they're always bent in half (I don't think they've always got ginormous fish hooked up on every rod. except for the guides of course).

Is that tension between the rod and the cannonball? Or is it tension from the size bait being trolled, transferring through the weight and up to the rod? Does it somehow translate into a snap hook-set when a fish takes the bait?

Seems like a lot of pressure on the gear. Never been on a boat with it and just don't understand what's going on.