I saw an hour long special on CNN that explained how pressure cooker bombs behave. When filled with nails or other projectiles, they are know to blow off pants but leave no wounds.
When pressure cooker bombs explode, they actually implode; that explains the windows being sucked towards the bombs.
Pressure cooker bombs also have built in targeting and that is how only the mans legs were removed and why everyone within 25 feet of the pressure cooker bomb full of nails was not more seriously injured.
It was a pressure cooker bomb, the news told me so.
Also would like to add that those who think the scene is real seem to do a lot of name calling.