When you say "simple on or off" do you mean the on/off powering of the unit, or easy-mounting and -dismounting off the bike?

The Cygolite Hypershot 350 USB has:

• Steady mode
• Memory mode -- your last mode is remembered, so that when you start up the next time it's in that same mode (ie, mode=steady); so long as you don't change modes to one of the other 6 it'll remain in "steady" mode
• 350 lumens output -- 2hrs duration, at that "high" setting; wish duration on "high" were greater, but it's simple enough to ride with two of these units, and to use a power bank to charge-up the drained one while the other is running, for extended rides
• Very compact shape -- a bit wider than 1"W, about 2"T, and a bit over 1" thick (off the flat mount)
• A rear rack mount that's flat against the rack (to which the light mounts); and the light itself quickly disconnects from that mount
• While it sticks out from the rack a bit more than 1", it's very compact

Question: Why is it that "flat" is important? And, why is it that only 3/4" thick (off the mount) is important?