I googled it and found here an explanation: "Bloodstain pattern interpretation is a discipline, which utilizes the sciences of biology, physics and mathematics. . . . The study of bloodstain patterns and the consideration of the physical processes in the distribution of these patterns to reconstruct details of activities at scenes of violence have recently emerged as a recognized forensic skill. . . ."
"Low velocity impact [bloodstains include] blood transfer patterns caused by hair, swipes, hand, foot or bloody object impressions."
The document at the link is in pdf format, so I was switching back and forth typing a few words at a time and I restricted myself to the gist of it. (My eyes are crossing now!)