Note that when a labiovelar adjoins an /o/ affected by Cowgill's law, the new /u/ will cause the labiovelar to lose its labial component (as in núks and ónuks/ónukh-, where the usual Greek change *k? > p has not occurred).
Cowgill's law in Germanic has no relation to Cowgill's law in Greek other than having been named after the same person. It says that a PIE laryngeal /h?/, and possibly /h?/, turns into /k/ in Proto-Germanic when directly preceded by a sonorant and followed by /w/. This law is still controversial, although increasingly accepted. Donald Ringe (2006) accepts it;Andrew Sihler (1995) is noncommittal.
Examples are fairly few:
If it becomes generally accepted, the relative chronology of this law could have consequences for a possible reconstructed phonetic value of h?. Since Germanic /k/ results from earlier PIE /g/, and since the change occurred before Grimm's law applied (according to Ringe), the resulting change would be actually h?w > g?. This would have been more likely if h? was a voiced velar obstruent to begin with. If h? was a voiced labiovelar fricative as is occasionally suggested, the change would therefore have been: w > .