Pep Guardiola had heard the stories and been warned of the threat. He had even studied footage of this weapon being deployed to prepare, but it was only when he saw the devastating impact with his own eyes that he realised its true power.
Manchester City's manager was stunned, a mixture of surprise and anguish on his face on the touchline on Saturday as he watched the bombardment. They do not teach this sort of thing at Barcelona. It is not a style of play the Spanish tend to spend much time pondering.
This was not football as he knew it and for a while, Guardiola looked scared, like a soldier hiding behind a tree under artillery fire.
In one memorable second-half exchange, with the scores level at 1-1 and with League Two Cheltenham preparing to take a throw-in, Guardiola stood wide-eyed, shouting at the match officials, vigorously complaining they were taking it a few centimetres further forward than where the ball had gone out.
Rarely has Guardiola looked so rattled, not in cup finals, title deciders or Champions League showdowns. This, though, was the fourth round of the FA Cup and it is why so many still love it.
City were not taken by surprise when Ben Tozer, the magnificent Cheltenham captain, took the ball in both hands, stepped as far back as the advertising hoardings around the Whaddon Road pitch would allow him, and then launched his throw-in like a rocket-propelled grenade into the opposition box.
They had looked at the different arcs of the ball. The long looping one and the flat skimming one. They had analysed the threat, done a full risk assessment and had worked on how to repel the attack.
A back line of elite defenders would have been told if you do not win the first ball, make sure the second wave of defence deals with the danger. Do not let the ball bounce, do not let the grenade explode in the area.
But it did and Cheltenham had scored, bodies everywhere, limbs flailing. Alfie May spotted the chance and seized the moment, smashing the loose ball home amid the panic.
Cheltenham Town were in front and, until the 81st minute, when a Phil Foden-inspired comeback began, were on course to pull off one of the greatest FA Cup upsets.
That City won 3-1, with Gabriel Jesus and Ferran Torres scoring the other goals, was not the most memorable aspect of this absorbing tie. It was Guardiola's face when Tozer got the ball in hand, it was Aymeric Laporte complaining he had been fouled as May celebrated and it was City sending on their big names desperately trying to find a way back into the game.
"Every one of those City players know they have been in a real game and that makes us so proud," said Cheltenham boss Michael Duff. "We gave them a fright."
They most certainly did and Guardiola was big enough to admit it. He had never played a team with a long-throw threat like this before.
"Unfortunately, in the Tony Pulis era at Stoke City, I was not here," Guardiola said. "But I heard a lot of stories about it and have been told by my staff about this weapon.
"It's an incredible weapon. It is more dangerous than a corner. You can avoid corners and free-kicks, but avoiding throw-ins is impossible. And when they put it there it's so difficult. We knew it, we saw the clips, we tried to solve it, but May is a smart guy, moving for the second balls and they scored a goal."
©Telegraph Media Group Limited, 2021
Telegraph Media Group Limited