Hecarim's passive makes for some very altered stats. At first sight, the scaling doesn't seem like much, especially if it's just off bonus speed, but it does have a role into how you play him. (or are supposed to) The scaling eventually just makes Hecarim have higher stats and makes him viable for different builds.
This ability, in my opinion, work well with two things: low attack speed-high damage; and trinity force(high mobility-medium-high attack speed).
The reason for this is that Rampage(q) interrupts your attack cycle, but has a very low cooldown w/ stacking reduction. At a peak of a 2 second cooldown, it can easily supplement your auto-attacks and overall damage, as long as it doesn't interrupt your attack cycle too much. However, it does not interrupt your movement.
If you have high-damage, low attack speed, there is less likelihood of the skill interrupting your attack cycle and also does more damage, overall increasing damage output in that scenario.
Trinity force will be explained later on.
To be Honest, this is one of the strongest non-ultimate abilities in the game. The fact that you can turn huge amounts of damage into health, even if for just a short duration, is extremely strong throughout the entire game. Laning phase is probably when it's most effective, allowing Hecarim to sustain himself through lots of harassment over time.
Yet this ability is more effective in team fights, when your carries will be doing the most damage. You can even heal from the damage they do, allowing you to heal massive ammounts from initiation bursts. This skill makes Hecarim extremely durable and sustainable. It also puts him in the top place for Aoe and Burst teams.
Timing, however, is key to this ability. You have to activate it just as the damage is coming in on the enemy, AND when your health is low enough to get a decent healing from it. The damage from this skill is pitiful, and only on rare occasions should it be used for damage. If the skill is not activated at the right time, it's potential is completely thrown away. However, if activated at just the right moment, it can mean the difference between a winning fight and a losing one.
There is not that much to this spell. It's like a rammus powerball how you gain movespeed as you continue this ability, and ends once you initiate an attack. You will knock an enemy back on the attack. However, you will leap-strike to them (quite literally), increasing your attack animation, attack range, and (at certain times) your attack damage. The damage ratios on this skill are poor, so it should really only be used to either knockback an enemy into your team, to chase down an enemy/initiate, or to run away. Many times have I tried to deal damage with this skill, only to end up wasting it and getting myself killed.
*note that on the knockback, you also travel a set amount of distance. Be careful you don't launch yourself into a bad position.
Another strong spell on Hecarim. It's pretty much a damaging dash with a small, but useful disrupt at the end of it. It is very strong for initation and has a very high damage at early levels for a ultimate. However, the end splash area is decievingly small, so be sure to aim the skill right. Aiming the skill slightly behind the enemy is best for maximum damage as well as a good position. Leading this spell behind a group of enemies allows you to knockback an enemy into your team, or any other direction you wish. I would prioritize aiming behind rather than getting the splash off in most cases.