in the other words,,. compressor will be controlled by another sound (key input), it means both sounds will not play simultaneously,. trigger will play and compressed sound will be affected by the adsr envelope of trigger.

just imagine adsr envelope of the trigger (ie kick). Trigger will hit and as sound is sustaining. compressed sound will go up almost on the same curve (reversed) as trigger sound.

curve shape will be affected by the settings on compressor on compressed sound.....parameters like,,ratio,thresh.. will do the compressed sound more go-ahead,,,or just gradual...

so.. i hope it's not too confusing,. and hope this is all true.. i just tried to describe my understanding of sidechain.. if i could talk in Czech,.,would be better, imo..