If you look at the scene where Jack interrogates Greg about his actions and wanting to marry Pam (after his episode on the plane near end of the movie), you will see that Jack has his thumbs on the inside of Greg's wrists to get the pulse. This is the incorrect way to get someones pulse as your thumb has a pulse also. So infact Jack wasn't testing Greg's pulse but his own. The correct way is with your index, middle and ring finger as anyone in medical field or interrogation field would know.