Two wires running parallel on a chip or PCB form a tiny capacitor through the dielectric between them. When the aggressor switches, it shoves charge through that mutual capacitance onto the victim — even though no current source is intentionally driving it. The victim glitches, slows down, or speeds up depending on what its driver is doing at that instant.

Two coupling mechanisms matter:

• Capacitive coupling (Cc): dominant at chip level. A rising aggressor injects a positive pulse onto the victim proportional to dV/dt × Cc.
• Inductive coupling (M): dominant on PCBs and packages. Current changes in the aggressor induce a voltage in the victim loop — this is why fast edges on power planes radiate.

The damage shows up three ways:

• Glitch (victim quiet): Aggressor switches, victim is being held by a weak driver, gets bumped above Vih or below Vil. Downstream gates see a false transition.
• Delay push-out (same direction): Both switching the same way — coupling actually helps the victim, it arrives early. Hold-time violation territory.
• Delay push-in (opposite direction): Aggressor switches against the victim, fighting it. Victim's transition slows by 20-40%. Setup-time violation territory.

Concrete example: DDR4 data buses run 8-16 bits in parallel at 3200 MT/s. Adjacent bits are aggressors to each other. When 15 bits switch low-to-high simultaneously and one bit goes the opposite way, that lone victim sees its edge slowed dramatically — this is the classic simultaneous switching noise (SSN) worst case. Memory controllers spec a "data bus inversion" (DBI) bit that inverts the byte if more than 4 bits would switch, capping aggressor count.

Rule of thumb: Coupling noise on a victim ≈ Vdd × Cc / (Cc + Cv), where Cv is the victim's total capacitance to ground. If Cc is 20% of Cv, expect ~17% of Vdd as a glitch. At 1.0V supply, that's 170 mV — more than enough to trip a gate with a 500 mV threshold if multiple aggressors gang up.

Defenses, in order of cost:

• Spacing: doubling wire separation roughly quarters the coupling. Cheap but eats routing area.
• Shield wires: route Vss/Vdd between victim and aggressors. Used on clock and reset nets.
• Twisted routing: swap victim positions periodically so every aggressor couples equally — net effect cancels (differential pairs do this naturally).
• Slew rate control: slow the aggressor's edge. dV/dt drops, coupled charge drops linearly.