Caesar's trick at Pharsalus. He created a 4th line of heavy infantry to his right directly behind his very weak cavalry. He was outnumbered across the entire front and has much thinner lines. His cavalry, while battle hardened and brave was very weak compared to the opposition (1800 vs 5000 a 8000) so the first breach would likely be that Pompey's cavalry would defeat Caesar's and then outflank his army.
Which itself couldn't really hold the line that long as they were outnumbered 2 to 1. Because Pompey did not advance, Caesar did. Then stopped just outside of javelin range to rest a bit.
Once they advanced again and joined the battle the massive cavalry of Pompey started their assault and quickly drove back Caesar's cavalry. It looked almost without much of a fight really.
In reality Caesar's cavalry feigned a retreat just when the cavalry of Pompey closed in a big mass. It revealed the 4th line of infantry (of the renowned 10th legion) and the Pompeian cavalry smashed into that line. That line had been ordered to not throw their pila, but instead use it as a spear against the horses. Chaos erupted among Pompey's cavalry and many lost their lives. The majority was still alive though but fled into the hills in a very disorganized fashion. That left the flank of Pompey's army exposed.
Meanwhile Caesar's cavalry was returning. But even before they were back the 4th and 3rd lines were ordered by Caesar to attack the now exposed left flank of Pompey's line and that quickly crumbled. Caesar's cavalry could successfully screen against any returning cavalry. Quickly chaos erupted on the Pompeian side.
Pompey judged the battle lost and escaped...
The trick was the way the very thin lines of Caesar's cavalry could hide the infantry directly behind them, some simply retreated through the ranks, others had feigned a counter attack and then wheeled behind the 4th line. As a result all of a sudden the Pompeian cavalry charged into a wall of heavy infantry which totally broke their charge and their spirit.
