Tap the display to cycle through Tracing and Fast modes. Tracing shows internal registers while Fast mode executes at full speed.
This is a microcode-level emulation of the HP-65 running the original 65 ROM. You can switch to HP-35 if you like. For wonderfuly detailed information about the internals of the HP-35, check out Jacques Laporte's site.
From your iPhone, tap [+] and "Add to Home Screen". When launched from there it will run full screen and fits nicely.
From a desktop machine, keyboard shortcuts are available (hover the mouse over a key for a hint).
For more info on how it was made, check out this blog. Please send bug reports to Francois Roulet
Here's the Opcode Map.