All it does is adds a speedup to the hero(Doesn't matter how you do it). Personally I would use Purge since you can make it increase a negative slow(resulting in a speedup).
Nextly you must make a dummy belonging to the caster to cast faerie fire on the target unit. This will be important for the next part.
Give the caster a bash spell that has a 100% chance to stun and set it to the level of the charge being cast.
Order the unit to attack the target unit(Put an appropriate wait since otherwise your spell will recieve no cooldown, .4 should do nicely).
Now you just need a trigger to detect orders given, and just remove everything that the charge adds(buffs/spells). Also you need a trigger that fires every few times a second that makes the unit's vertex colouring darker and makes their transparency higher.
This is(I realize) a cheaply done way, but it is the way that DotA does it.
Enjoy