Rumil, are you in anyway an aficionado on cavalries?
Battlefield Comment
If the hillock was close to the Entwash and carefully selected by Eomer as the place to herd the orcs onto, then I would think that the Marshal of the Mark had decided on a cavalry charge as his coup de grâce already when he intercepted the enemy. For the cavalry charge to be effective, the cavalry commander must prevent his target from shifting because once committed, the impetuosity and inertia of the charge would make a quick change in direction impossible. If the enemy is massed in a large group of several thousand bodies or so, its own bulk and inherent rigidity would prevent it from dispersing rapidly to avoid the incoming shock. But in this case there are only 200 hundred or so orcs - very easy for them to scatter in all directions and render the cavalry charge impact void. Something was needed to helm them together, to prevent them from avoiding the horses and I belief the Entwash was that barrier.
Any cavalry vs. infantry engagement is more of a contest of wills than brute melee and shock is the key. An for every infantryman, nothing is more terrifying than facing a cavalry charge with a wall behind his back to prevent him from escaping. Once the pressure exceeds the infantry's threshold of tolerance, the will to fight snaps and the unit routs.
I hereby submit that Tolkien had the "anvil and hammer" effect in mind when he wrote of the battle. Eomer and his eored was the hammer while the Entwash was the hammer. Provided that the Entwash was in close proximity to the hillock i.e (<100m distance).
Charge Comment
I tend to believe that the Rohirrim Cavalry charged pass the gaps between the orcs as opposite to through them literally, crushing all beneath hoof, simply because as stated by Fuller and Keegan, NO heavy cavalry had in the course of history from antiquity to the Great War ever done so.
As mentioned, anti-cavalry warfare was essentially a psychological duel. Units that panick, break and scatter would be those that cavalry "overrun" After the Napoleonic Wars had ended, the British Army conducted a study and discovered that the only time that a resolute square of infantry was broken by cavalry alone, was when the dead and out of control carcass of a charger crashed into it and nearby horsemen immediately exploited the gap created. In no instance during the countless breathtaking cavalry charges that took place did cavalry mow on and flatten all before it through sheer momentum and weight. Not even during the mass cavalry charge at Eylau where, it is now more or less accepted that the Russians rout and parted ranks when the French heavies reached them. Makes sense because it would have been impossible for Murat and company to wheel around and charge back to the French lines had they bloodied and squashed their way through the entire Russian depth in two feet of loose snow.
Georges Blond, the premier French Historian on the Grande Armee placed the average cavalry charge at only 18km/hr and at Eylau due to bad weather and ground conditions, it was even less. That meant that despite mass, the French cavalry lacked the momentum to physically overrun the enemy.
Another way to decide if a cavalry did literally squash all in its way underhoof is by counting the cavalries casualties. The logic is that no matter how fast and heavy, the cavalry is bound to suffer heavily if it keeps crashing into things constantly. Throughout his entire Persian Campaign, Alexander's Companion Cavalry suffered less than 10% casualties as compared to his other arms and at the Battle of Patay, La Hire and Poton de Xantrailles lost only an estimated five men in their charge through English ranks (the English lost 2000 over men).
Another thing to note is that unless a rider delibrately directs his mount towards an obstacle, both rider and horse tend to avoid obstacles instinctively and subconsciously.