There's supposedly some "alternative" way to make it zoom in/out by holding and then moving up/down, but I can't get it to work.
To zoom with one hand, do a double-tap, and upon the second tap, hold. Then slide down to zoom in and up to zoom out.
i.e. tap, tap+hold... (while holding) slide up/down
"Pay no attention to the man behind the curtain." -- The Wizard Of Oz