Colloquially, it's basically a change in voltage in one place can indirectly cause a change in voltage in another place via capacitive coupling. Capacitance increases proportional to the inverse of the separating distance so only in recent years have things shrunk to the size that makes it an issue.
Since having less bits in DRAM is basically not an option most mitigation techniques that I know of remove the possibility of hammering: possibilities include the OS, memory system controller, or DRAM controller changes.
[1] https://users.ece.cmu.edu/~yoonguk/papers/kim-isca14.pdf
They should also have logic to refresh adjacent rows if some number of consecutive accesses to a small group of rows is detected. This is rare in normal workloads, because those accesses normally come from cache. It's lame of chipmakers to not fix this. The fix would requires the DRAM controller (integrated into modern CPUs) to know more about the internals of DRAMs than they currently do.