What is the lingering doubt penalty defense?

Defendant contends his attorneys were ineffective because they "failed to present the issue of lingering doubt, a relevant factor in mitigation." We have noted that "the sword of the lingering doubt defense is double edged. (People v. Fauber (1992) 2 Cal.4th 792, 864, 9 Cal. Rptr.2d 24, 831 P.2d 249.) Because such an argument risks `"antagonizing a jury that has already found the defendant guilty,"' lingering doubt penalty defenses '"are often unwise.'" (Id. at p. 864 [9 Cal.Rptr.2d 24, 831 P.2d 249].)" (People v. Padilla (1995) 11 Cal.4th 891, 951, 47 Cal. Rptr.2d 426, 906 P.2d 388.)