C4 has a detonation velocity of 8,092 m/s (26,550 ft/s). The explosive in C4 is RDX which makes up around 91% of C4 by mass.
To set off the explosion, RatedRR used detonating cord, a thin flexible plastic tube filled with PETN. The PETN explodes at a rate of about 4 miles per second. It's a high speed fuse which explodes, rather than burns.
The detonating cord was run in the shape of the Apple logo and slow motion cameras captured the explosion as it moved along the cord.
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*If you like these types of videos check out this iMac vs a 20mm Anti-Tank Rifle video.