/diy/ - Do It Yourself

Hello /ohm/, I'm doing a project where I use an attiny and an optocoupler to drive a speedlight, which is supposed to fire when the signal on the cable is shorted
The problem is I'm having trouble finding information on how to wire the optocoupler: I'm using a 4n34 and I've found pic related online (I know it says cn-17 but the 4n35 has the same schematic)
As you can see there's a pullup resistance on the base, but I've found people putting a pulldown, while others leaving it floating.
I have basic knowledge of NPN transistors but I don't get how this is supposed to work...