[Smokin' Joe]

A lot depends on the target.

If the busses were the target, a motion activated switch or a timer, If the crowd (but not himself), a timer or remotely detonated device would be effective. For a suicide bombing, a mechanically closed switch (command detonated or 'deadman') would probably be the ideal.

Remotely activated detonators would have applications, too, but could be set off on some dupe to eliminate the basic witness.

A lot would depend on whether the bomber was working alone, whether he was the top guy or a "soldier" and the target.

I imagine any unexploded ordnance recovered will help answer those questions and help discern his intent, place in the pecking order, likelihood of working alone, etc.

It is no doubt being thoroughly studied while we speculate.

[Robert A Cook PE]

Reverse that:

A "deadman" switch does the opposite: A "live switch" (regular one) would be pushed down, establishes contact, current flows, and the explosive goes off. You need a "live man" to make it go boom.

A "deadman" switch is held down by the bomber, when/if he is killed (or lets go of the switch to scratch his ear) he releases the handle, which makes contact inside the switch, which establishes current, which make the bomb go boom. There are lots of flavors or options with these: adding timers, adding remote detonators (in addition to the ones the bomber knows about) to ensure a kill, etc.

This supposed engineering "student" was probably duped into playing his suicide bomber game by much more serious players. Or he simply screwed up: Had a deadman switch, but let go of the handle while waiting for the busses to get filled nearby.