[BR3W CITY]

Ignition switches on older vehicles are sometimes run that way, and it can start the truck but you are essentially making the juice for the starter solenoid go all the way from the battery to the fuse then the ignition switch, then down through. This puts all the current (and heat) through small wires. The ign switches on older cars/trucks are heavier and more capable of this, especially when compared to things like push-start switches which are NOT heavy duty at all. The similar principle applies to headlight wiring, using a relay vs hard wiring through the switch.

Modern cars will use the ign switch to carry a signal to the input side of a relay. That relay can then be wired with a heavier wire, and creates a barrier should there be a higher current short at the starter. OR they can use the PCM to send the signal to the relay/starter, depending on the year/type of setup.

Lockup is when the convertor literally locks up at a 1:1 ratio to prevent slip and ideally minimize heat.