- On (obsolete) coax cable, a collision is detected by the channel voltage exceeding the normal scope (less than -1.5 V, see IEEE 802.3 Clauses 22.214.171.124 and 10.4.1.5).
- Some stations detect a collision sooner than others - the overlap of concurrent signals is local to each station, due to the limited propagation speed of the signals.
- Concurrent signals interfere with each other and produce different voltage levels all over the bus (running in different directions with limited speed). To make sure that all stations sense a collision, a jamming sequence makes sure that it's propagated throughout the collision domain. E.g. when two stations close to each other produce a collision, a station further away might not detect it due to the (short) signals not overlapping at its location.
- Yes, the jamming signal collides with a possibly still continuing transmission - one of the transmitting stations might not have sensed the collision yet. The transmitting station needs to monitor its tap for voltage anomalies.
Note that collision detection is simpler, more reliable and faster for full-duplex capable channels like fiber or twisted pair (channels, not links which would mean switching). Since there are dedicated send and receive channels, the sender simply needs to monitor its receive channel for a carrier, signaling a collision. That is the exact reason why the 5-4-3 rule allows more link segments (point-to-point) than it does mixing ones (multi-tap).
For reference, IEEE 802.3 defines jamming this way (the exact jamming sequence is defined in each PHY):
126.96.36.199.4 Collision detection and enforcement (half duplex mode only)
Collisions are detected by monitoring the collisionDetect signal
provided by the Physical Layer. When a collision is detected during a
packet transmission, the transmission is not terminated immediately.
Instead, the transmission continues until additional bits specified by
jamSize have been transmitted (counting from the time collisionDetect
went on). This collision enforcement or jam guarantees that the
duration of the collision is sufficient to ensure its detection by all
transmitting stations on the network. The content of the jam is
unspecified; it may be any fixed or variable pattern convenient to the
Media Access implementation; however, the implementation shall not be
intentionally designed to be the 32-bit CRC value corresponding to the
(partial) packet transmitted prior to the jam.
Of course, coax cabling and half-duplex communication are all but obsolete. Modern networks use full-duplex, fully-switched links throughout.