Blocking is for the weak. Real men run away and throw javelins. Real men used to run away and throw rocks, back in my day.
The "Combat State" thing has some specific animations that go with it, if you pay attention you have probably noticed them before. It engages when you ready an attack. The devs discussed it in some of the version update discussions, and if memory serves there's an additional multiplier in Captain Mode for Skirmishers for it. It basically means you can disengage from any infantry fight and nobody can immediately stop you. Some Shocks are faster than you and will catch up to you again, but you are able to juke out of the way faster than they are after a block. Unfortunately the AI cannot take advantage of this, and you will notice that one or two of them will peel off and engage pursuers, while the rest of your guys that are not "engaged" are able to keep moving.
Skirmishers are really the opposite of Shock Infantry. Shocks can do massive bursty damage but can't take it. Skirmishers can usually take slightly more punishment than lights, but take a long time to dish out damage. You can output enough damage to kill a couple light infantry or a heavy infantry in your javelin payload per skirmisher, but it takes a long time to throw them all. In my experience, Skirmishers are most effective when you are hanging out near your allies, screening your archers from melee charges by hanging out in loose formation and pelting incoming troops. Wildlings in particular excel here with their Clubs, since what makes it to your Archers or Heavies is usually enemy Heavy Infantry.
You are in a prime position to take advantage of any mistakes your opponents make, though. You're faster than anything except Savages, Zerks, and Cavalry. You can break off from your allies to punish positioning mistakes, but you can't usually finish any fights on your own and have to return to the main body (or get cav/archer support) if you meet any real resistance.
A tactic I'm pretty fond of is False Retreats with skirmishers. If you have heavy allies nearby, or coming up to support you, you can engage opponents very briefly on a Charge with throwing weapons, then use the Fallback command to have your troops flee in many directions.
If your opponent is foolish or not paying attention, they'll leave their troops on "Charge" and get spread out. If your allies are paying attention, they can smash right into the center of the stretched enemy, you set your guys to Charge as well, and you just mop up.
In general, outputting a lot of damage to stationary heavy infantry with Skirms is hard. If you can, be on the right-hand side of an opponent's formation, as you're more likely to find a space without a shield in it. Historically, this is also how Peltasts functioned, and why modern armies line up with the highest ranking guy on the right-hand side of the formation. It is a place of pride, as they knew that's where all the javelins would be targeting and you must be a total badass to volunteer for that.
Your job is really to punish unshielded enemies in bad spots, soften up and spread out enemies, then come in behind your allies as they tank damage.