How does it work??

[charlie_ps2]

Where to start? You have the Messiah 2!

An original PS2 game is pressed onto disk. The pressed disk contains a deliberate physical wobble in the pre-groove area that induces a tracking error when the laser has focussed onto the groove. The nature of the tracking error is reported by the receptor diodes and after decoding via the DSP the info so gleaned is compared with the region information for that PS2. By this coded approach, Sony determine whether or not the disk is a game intended for that region and passes to the next stage, acceptance or rejection (Red Screen).Each region's PS2 contains it's respective region code and games for each region has appropriate encodement in the wobble.

A DVD-R cannot have a wobble in the groove. When the disk is read by a DVD-R burner, the tracking error is auto-corrected. So when you boot a DVD-R without a modchip, the region test fails because the wobble algorithm test fails.

A no-swap modchip is developed by people who know how this works in detail. So a number of wires are attached to the PS2 (they've discovered which points) and the modchip then looks for and replaces events of interest so that the disk in the drive is passed as OK.

if it is OK (apart from the groove wobble) it boots; if it's not OK, it goes black screen or Red Screen depending on how much not OK.

That's answered your question.