How can I use built-in effects processor that goes to main to send to monitors?
Some Behringer mixers have dedicated mon fx sends like the below (from the PMP1600):
Other Behringer mixers (e.g. the XENYX X1204USB) don't have that option and here you have to patch back the FX send into an unused channel and from this channel send to the monitors in question (but DON'T send to the FX send - it will create a feedback loop!).
Note that Aux Return 2 often used as a way to disable the internal FX processor, so make sure you don't put anything into that (source, p. 5)
I just wanted to add a bit more details to Michael's answer regarding how to make it work with the UFX1204. I know it's 5 years later, but the manual doesn't properly address this trick.
First, send channels you want to hear effects on by turning their individual FX knobs. Press the FX ON button as well in the MULTI-FX section.
Then send everything on the FX bus to the mixer's FX processor by turning the FX SENDS knob inside the SENDS/RETURNS section.
Once the signal is processed by the FX processor it's returned to FX RETURNS. The amount that goes back into the main mix is controled by the FX RETURNS knob just next to the FX SENDS knob from the previous step.
Now we don't want FX RETURN to only go to the main mix, we also want it sent to monitors. To do that, press the "TO AUX 1" button. The little arrows tell you that doing so sends FX RETURNS not just to the main mix, but to AUX 1 SENDS as well, so adjust the red AUX 1 SENDS knob too.
What we've done so far is to send all channels that are processed through the FX processor to AUX 1 SENDS. So take a TS (or TRS) cable and patch AUX SENDS 1 at the back of the mixer to an unused channel and that's it!
If you now SOLO that channel, you'll hear effects coming out of the FX processor. As Michael pointed out, don't turn the FX knobs on that return channel or you'll create positive feedback..or carefully do turn that knob for some added fx crazyness. For exmaple with a little feedback you can get not just one but two subharmonics using pitch shift turned all the way to the left.
SOLO both that channel and the channels that are being sent to the FX processor to hear the sum that's going out to the main mix. No more annoyed neighbors.