In my opinion, you have to know how to open direct or indirect if you want to open any girl/group you want. Direct and Indirect approaches are not different games but part of the same theory.

PU is all about compliance escalation.
The initial level of compliance that the target gives you is all about your passive value (look, social proof, leading other sets,...). If you have very good passive value and social proof, if you seem to be THE man to know in that place, your target is gonna give you some IOI (warm approach). If you have not enough passive value to already attract your target, you are going to face the famous "bitch shield" like others men of low or indeterminate value.

That's the core of "forcing IOI". Forcing IOI is nothing but a compliance test before the approach, to know where your target is on the compliance ladder and how to calibrate your game (direct vs indirect). When you are in (warm-up) sets, look at another potential target and raise your glass or make a sign. If she responds positively (IOI), you know that you can open direct or situational, you are already on A2-A3. If she doesn't respond well with a non-verbal "boy you are out of your league" (bitch shield) you know that you have to build more value, in exemple by opening her set indirectly (DHV and neg her): you begin on A1.

ps: I apologize if my english is not so good