MODE 1 ("Load alt. core"):
Load the EE core at LOWER address (0x90000) but uses a g_buf (used for IOP resets) at higher address (0x1700000). The normal core loads at 0xe80000 and uses a g_buf at 0x88000. It is used because some games are overlapping normal core memory area (Sonic Unleashed loads at 0xd0000).
No speed difference caused by this mode.
This mode fixes some games that freezes on green screen
MODE 2 ("Alternative data read method"):
cdvdman module uses an alternative method to read data, not using semaphores.
Some games requires it.
MODE 3 ("Unhook Syscalls"):
Similar to HDL's mode 3: it disable IOP resets hooking.
MODE 4 ("0 PSS mode"):
This mode tries to make .PSS file some dummies. This mode help to run Max Payne through USB (timing issue with videos causing crash), and can be useful if the game contain a lot some PSS file you want to skip.
This mode fixes some games that freezes due to timing issue with videos
This mode can be used to skip PSS videos
MODE 5 ("Disable DVD-DL"):
A bit like HDL mode 2, it disable the DVD DL support, Jimmi implemented this mainly because to handle DVD9 it read after last ISO sector, to see if it find a 2nd layer.
With the eventuality the last sector of the ISO is also the last physical sector of the disk it could freeze.
MODE 6 ("Disable IGR"):
If activated, the In-Game-Reset functionnality is disabled. Use this mode for the few incompatible game with IGR.
MODE 7 ("Reduced cdvdfsv buffer"):
Reduce the buffers used by cdvdman/cdvdfsv. This helps to solve the problem with game being short in IOP memory (usually in SMB mode), ex: ICO (SCUS_971.59), Disgaea, ...
MODE 8 ("Hide dev9 module"):
A bit like "Disable Network support" mode of some HDL versions. This mode is needed for online gaming with some games (NFS Underground 2)
UDMA/MDMA MODE: Only concerning the IDE HDD, their purpose is to set the transfer mode that will have an impact on speed.