An undercut is a pit-stop tactic used to pass a car you are stuck behind on track. Instead of waiting, the chasing driver pits first and switches to fresh tyres a lap or two before the car ahead.
Fresh tyres are faster than worn ones, especially in their first few laps. The freshly-stopped driver puts in one or two very quick laps while the rival is still circulating on older, slower rubber. When the rival finally pits, those gained seconds can be enough for the undercutting car to come out in front.
The undercut works best when fresh-tyre pace is strong, the pit lane time loss is low relative to the lap-time gain, and traffic does not get in the way of the crucial "out lap". It is most powerful in the middle of a stint, once tyres have started to degrade.
The counter-move is the overcut: staying out longer than a rival while they struggle on fresh tyres that have not yet warmed up, or while they hit traffic, then pitting later to emerge ahead. Teams weigh both options live, lap by lap, based on tyre wear and the gap behind.
There are limits and risks. A pit stop costs a fixed chunk of time — the pit-lane delta, typically in the region of 20-25 seconds depending on the circuit's pit-lane length and speed limit — so the fresh-tyre gain has to outweigh that loss for the undercut to work. The classic danger is the "undercut trap": the car ahead may have just enough margin, or enough remaining tyre life, to respond by pitting the very next lap and keeping the position. A failed undercut can also drop a driver into traffic, ruining the out-lap they needed to make it pay. This is why teams watch tyre degradation curves and the gap behind so closely before committing.