What does “stroboscopic flash” mean on the Canon Speedlite 430EX II?

When Canon refers to stroboscopic flash, it is in the context of multiple flashes produced during a single exposure at shutter speeds slower than the camera's Flash Sync Speed. This technique, also referred to as MULTI Flash, is not available with the 430EX II unless it is used as a Slave flash to a Master flash that has this feature. Such master units would include the 580EX II (see page 46 of the 580EX II Instruction Manual), and the 600EX-RT. Unfortunately, other than the hint on page 37 that this question addresses, this capability when used as a slave unit is not documented anywhere in the User Manual or promotional materials for the 430EX II!

Incidentally, Canon refers to multiple flashes used to expose the entire film/sensor plane at shutter speeds above the camera's Flash Sync Speed as High-Speed Sync. This feature is available when using the 430EX II.

A statement similar to the one at the end of the appendix on page 37 of the 430EX II Instruction Manual appears in all of the manuals for Canon flash units that list which Canon E-TTL features work with the older Type B TTL camera bodies and which are only available with the newer Type A E-TTL and E-TTL II bodies. To the best of my knowledge, all Canon Digital SLRs are Type A E-TTL/E-TTL II compatible.