Mind Flay is sort of like a chain of energy that the hero channels into the target's mind, slowing their movement speed and damaging them every 2 seconds for 6 seconds. Similar to the Siphon/Drain spells, if the target leaves the radius, the spell will stop channeling.
In terms of WoW, it is usually used to slow down a runner while damaging them at the same time as the warriors charge in and kill them.