Yes, it the gap between the piston and head; it is over just part of the piston surface. it needs to get down below .060" to even start having an impact and something like .035" is pretty darend good. Too tight and you risk the piston hitting the head as everything wiggles around at high RPM.
The idea is two-fold:
- The tight compression of the gases to the piston head allows the piston head to pull some heat out of the compressed fuel-air mixture, and so makes it less prone to detonation. That is the strict definition of quench.
- It also squishes the fuel air mix out of the tightening gap at high speed, and that promotes swirling and mixing of the fuel-air mixture in the chamber and gives a more complete and faster burn. This is known a 'squish' but we tend to lump that in with quench and call it all 'quench'. The faster burn makes the mixture less prone to detonate. (BTW, any time you speed the burn rate up, then any tendency to detonate decreases. which is why closed chambers tend to detonate less than open ones.) The benefit of the more complete burn is obvious: more power and efficiency (including better mileage and lower emissions).
If you are seeking optimum performance, you plan this in. You can run higher compressions and/or more advanced timing and still have better detonation resistance. The added compression and timing mean more torque and power and efficiency.
There is no argument that this works; it has been common in engine design since the 40's-50's and used in hot-rodding since the 60's, my 1st engine in 1975 had quench-dome pistons. The only valid arguments are if it is worth the trouble to the individual; it limits parts selection and often invokes more machining costs. I value it for being able to run more compression, which means more low RPM torque on a street engine, which is of great value in that use. I also like the better detonation resistance; I can be lazier in setting timing LOL
