One thing us dota nubs seem to understand better than starcraft players is the importance of slow/stun. Starcraft has two stuns (lockdown and maelstrom) and one slow (ensnare). In DotA, these abilities would be used endlessly for their disable power. But Starcraft hardly uses any of them... for reasons that are at best irrational.
In ZvT, there are several things to consider:
1.) Lurkers are one of two AoE units you have, and by far the most effective: however, their attack is slow and therefore easily dodged.
2.) Darkswarm is powerful but dodgeable. Keeping the enemy contained near the darkswarm makes it far more effective.
3.) Because Terran AoE is imba (tank splash, mines, firebats, valkyries) you HAVE to flank.
4.) Terran MnM have the ability to increase movespeed and attackspeed for 2 mana (the cost of a medic to heal the 10 hp loss). But the most relevant in ZvT is the movespeed, since it allows Terrans to dodge the lurkers and retreat from the darkswarm.
Ensnare solves all this by...
1.) increasing the ability of Lurkers to strike hits.
2.) keeping the opposing forces from running away from darkswarm.
3.) clumping Terran forces so they are easier to flank.
4.) And it does this by eliminating the movespeed bonus from stimpack, something you HAVE to do given how costeffective stimpack is for Terran and that they still get the attack speed bonus from it.
Compare the following two clips:
Flash vs GGplay, MST, December 6 2008
In this clip, Flash's marine split allows him to avoid the flank and incur minimal losses. Flash goes on to crush GGplay
Jaedong vs Fantasy
Fantasy tries to do the same thing from the incoming flank, but ensnare stops him.
notes on the clip:
1.) The initial darkswarm forces terran to reposition its troops.
2.) Ensnare slows down the repositioning process. This is quickly followed by Zerg's flank.
3.) The slowed retreat away from the darkswarm gives lurkers time to be well positioned. Terran's MnM have no chance to avoid the lurker spikes.
4.) Important ancillary effect: Medics, also being slowed, cannot heal effectively because they cannot catch up to the units that are damaged.
5.) Science vessels cannot avoid the scourge, tanks have trouble repositioning.
maybe this was a best case scenario usage of ensnare... but it still demonstrates the awesome power of slowing your opponent in zvt. in short: Zergs, use ensnare!