First off, one uses a Q2 and the other Q5, but I am fairly certain you already knew that.
The L1D had a reverse clicky, the L1T has a forward clicky (as currently sold by Fenix-Store, some dealers might still sell reverse clicky type)
L1T:
When the head it loose, the light operates as a single mode light in one mode, the low mode. No combination of switch pressing will get you any other modes.
When the head is tight, the light again operates in one mode, high mode. This is similar to how the NDI operates. Depending on what position the head is in, you have one of two modes with a forward clicky.
L1D:
When the head is loose, you are in "general mode". Turn the light on and it is in low. A soft press advances the levels from low > medium > high > SOS. There is no memory, leave the light off for a couple seconds and it will always turn back on in low mode.
When the head is tight, you are in "turbo mode". Turn the light on and it's in turbo. A soft press advances the levels from turbo to strobe. Those are the only two settings in this mode. No memory setting, it will come on in turbo by default.
For both lights, there is no complicated twisting of the head back and forth to get different modes. Think of the L1D as a regular multi-mode light where you have to soft press to get your different modes, but it has two different menus. The position of the head determines if you are using a 2-mode light, or a 4 mode light.