This is a story originating in the supposedly biographical work
岳鄂王行實編年, "Chronicles of the Life of King Yue of E". It was written by Yue Fei's grandson, who was trying to rehabilitate his grandfather's reputation. Predictably, he weaved a ton of exaggerations and outright fabrications into the narrative.
According to the conventional story, the Jin cavalry supposedly and inexplicably tied their horses together in groups of three. Yue Fei supposedly was the first Song commander to notice this, and ordered his soldiers to attack the horses' legs. For this, he supposedly invented a special weapon called
鉤鐮槍, literally, "hook-scythe spears".
Of course, that is almost entirely ahistorical. Yue Fei was not the first Song commander to defeat the Jin cavalry, and the Jin cavalry almost certainly did not tie their horses together - even the name given to this bizarre arrangement was lifted from a legitimate Song cavalry command.
The hook-scythe spears were historical, but has been in use against traditional light cavalry for hundreds of years before Yue Fei. It is basically a spear with a hook, designed to essentially trip the cavalry over by hooking the horses' legs. A picture is worth a thousand words:
In the original story concocted by Yue Fei's grandson, the Song infantry adopted a new anti-cavalry tactic, where they ignored the rider, kept their heads down, and used sabres with extended handles (halberd) to cut off the horse legs. This story was copied by the Mongolians when they wrote the History of Song, some two hundred years later.
In the battle, a 15,000 cavalry continent attacked. Yue Fei ordered his solders to engage using long handled sabres, and to not look up but instead focus on cutting off the hooves of the horse.
- History of Song, Biography of Yue Fei
In contrast, the contemporary and far more credible History of of Jin mentions no such thing.
The History of Jin went through several major revisions over 82 years before its publication. In contrast, the History of Song is an exceptionally rushed and low quality work, and should not be trusted. Unfortunately, as a government issued history of the previous regime, it help popularised the propaganda Yue Fei's well-intentioned grandson started.