(BG Spoilers) LTW Conservation of Speed w/ Mass Shift

[Arjan]

No this is magic. Physics need not involve itself. Jim even uses the fact in Cold Days in Harry's fight with Cat Sith on the boat.It's magic and you can't make it work as anything else.

Jim uses a combination of magic and Newtonian mechanics here. Both have their function in the story.

[Bad Alias]

They control their speed and angle of decent, something an elephant could never do.

Someone's clearly never seen Dumbo. 

[TrueMonk]

When you change from a hawk to an elefant I would say you go from flying to jumping, you can't really steer, but if you aim is good then it is fine.

I guess he could either be projecting his own force to keep the speed the same or be drawing in mass that is already moving, like grapping an apple while driving a bike, from another guy who is also driving a bike.

If Harry takes shape shifting lessons from River Shoulders maybe we will find out which one it is.