Why Full break-in, signaled by the Q signal QSK, means the transmitter and receiver switch fast enough that the transmitting station hears the frequency in the tiny gaps between dits, dahs and characters. That lets another station interrupt mid-transmission, which is handy for breaking in with urgent traffic or for hearing QRM develop on a busy frequency. It requires fast T/R switching, typically PIN diodes or a fast relay, rather than a slow VOX-style hangtime.
Watch out Sending the prosign BK is semi break-in style courtesy, where you pause and toss it back; that is not the same as hearing between elements. Manual send/receive switching is the opposite of break-in operation entirely.
QSK = hear between the dits. Break-in listens in the gaps, not between overs.