Background Although entrainment mapping is an established approach to atypical atrial flutter ablation, postpacing intervals shorter than tachycardia cycle length (difference between postpacing interval and tachycardia cycle length [dPPI] <0 ms) remain of unknown significance. We sought to compare anatomic and electrophysiological properties of sites with dPPI <0, dPPI=0-30, and dPPI >30 ms. Methods We studied 24 noncavotricuspid isthmus-dependent macroreentrant atypical atrial flutter in 19 consecutive patients. Ultra high-density electroanatomic activation maps were acquired with a 64-electrode basket catheter. Entrainment mapping was performed at multiple candidate sites. Ablation was performed at the narrowest accessible slow-conducting critical isthmuses. Results Of 102 entrainment mapping sites, dPPI <30 was observed at 72 sites on complete maps of 24 atypical atrial flutter. Compared with dPPI=0-30 sites (N=45), dPPI<0 sites (N=27) were more commonly located within isthmuses <15 mm wide (67% versus 6.7%, P<0.00001; odds ratio, 28.0; 95% CI, 6.8-115.7), more frequently located within 5 mm of the leading wavefront (93% versus 64%, P=0.008), exhibited slower local conduction velocity (0.49±0.43 versus 0.93±0.57 m/s, P=0.0005), lower voltages (0.48±0.79 versus 0.92±0.97 mV, P=0.04), and more frequently fractionated electrograms (67% versus 24%, P=0.0004). High rates of arrhythmia termination or cycle length increase >15 ms by ablation were observed in both dPPI groups (94% versus 86%, P=0.53). Compared with all dPPI <30, dPPI >30 sites (N=30) were less commonly observed within isthmuses (3.3%, P<0.001) or within 5 mm of the leading wavefront (30%, P<0.0001); conduction velocity (1.0±0.7 m/s, P=0.002) and voltage (1.1±1.4 mV, P=0.049) were higher compared with dPPI<0 but similar to dPPI=0-30 sites. Conclusions In atypical atrial flutter, sites with dPPI <0 are markers of limited width critical isthmuses with slower conduction velocity, whereas sites with dPPI=0-30 ms are often not in close proximity to the reentry circuit. Virtual electrode simultaneous down and upstream (antidromic) capture of a confined isthmus of slow conduction can explain a dPPI <0. Identifying these sites may improve selective and efficient ablation strategies compared with the standard 30-ms threshold.