GREEN BAY, Wis. -- Jordy Nelson doesn't want you to know everything about the ins and outs of the back-shoulder play, except he's quick to point out one thing.

"First and foremost, it's not a play," the Green Bay Packers receiver said. "For us, it's a complete reaction."

There is no signal or pre-snap adjustment.

Not even Nelson, one of the NFL's best at executing the play -- check that, the pass -- knows it's coming until the ball sails off the hand of quarterback Aaron Rodgers.

"It's all on the quarterback and what they decide to do and where they want to place the ball," Nelson said. "For us, it's just, run our routes. The main thing would be to make sure you get your head around and are able to adjust to the ball."

If the receiver doesn't even know it's coming, what's a helpless cornerback or safety in coverage supposed to do about it?

"If you have a quarterback that can throw it, the quarterback typically throws it at the defender's head," Packers cornerbacks coach Joe Whitt said. "So you never see the ball."

It goes against everything coaches like Whitt teach their defensive backs, whose first priority is always to take away the deep throw. The back-shoulder fade -- which is essentially a purposefully underthrown pass -- is the perfect counteraction to a defender intent on taking away the go, or fly, route.

"As a secondary player, you're always taught to be on top of a route, control the deep routes," Packers safety Micah Hyde said. "When you get on top, to have to react to a ball that's underthrown, that's the hardest thing for the DB."

But not just any receiver can pull it off. The bigger and longer the receiver, the easier it is to execute. At 6-foot-3 and 217 pounds, Nelson might have the perfect build to do so. It's why you won't see Rodgers throw it very often -- if ever -- to his No. 2 receiver, Randall Cobb, who at 5-foot-10 and 192 pounds is better-suited to play in the slot.

Then there is the experience factor. Someday, rookie receiver Davante Adams (6-1, 215) might see that throw from Rodgers. But until Rodgers knows Adams can react to the ball in the air the way Nelson can, it's not likely to happen.

"It comes with experience," Hyde said. "You've got to have chemistry with the quarterback, because it's a hard throw. To throw the ball before the receiver even stops, it's hard. I played quarterback back in the day [in high school]. It was hard then, and I'm sure it's 10 times harder now with these fast receivers. You've got to have a lot of chemistry, and I think some guys in the NFL, a lot of guys in the NFL, don’t have that experience that A-Rod and Jordy have."

As if it wasn't difficult enough to defend, oftentimes it comes with a subtle shove from the receiver.

"And boom, it's never being called," Whitt said. "It's a good play."

Except that it's not a play.