Growing supermassive black holes (SMBHs) are believed to influence their parent galaxies in a negative way, terminating their growth by ejecting gas before it can turn into stars. Here we present some of the most sophisticated SMBH feedback simulations to date, showing that the effects of quasars on galaxies are not always negative. We find that when the ambient shocked gas cools rapidly, the shocked gas is compressed into thin cold dense shells, filaments and clumps. Driving these high-density features out is much more difficult than analytical models predict. However, in this regime quasars have another way of affecting the host – by triggering a massive star-formation burst in the cold gas by overpressurizing it. Under these conditions SMBHs actually accelerate star formation in the host, having a positive rather than negative effect on their host galaxies. The relationship between SMBHs and galaxies is thus even more complex and symbiotic than currently believed. We also suggest that the instabilities found here may encourage a chaotic active galactic nucleus feeding mode.