Every automatic I've ever seen allows you to shift to a lower gear.
Engine braking works well when you rev-match as you downshift, then when in the lower gear simply let off the gas completely. You will slow down, and at that point your drive/axle is pretty much directly connected to the engine which is simply turning without gas being injected into it, thus using the compression cycle to slow you down.
In a manual car you don't want to shift to 1st gear when you are driving 100. That's bloody insane (Whether that is 100 km/h or 100 mph).
I think you think I was saying that you switch to 1st and let the clutch come up until you stand still, but that's not what I meant.
When you are in 5th, you downshift, while the clutch is still disengaged you blip the throttle to get it up to the required RPM's (in my car 3k in 5th is about 5k in fourth), then let the clutch up (no slipping, RPM (rev) match it), at this point you are no longer giving the engine gas. It will start to slow you down, from 5k back to 2k, disengage the clutch, blip the throttle to 4.5k, 3rd gear, let the clutch up, slow down further.
The same way that you have to rev-match when down shifting to go pass a truck on the highway quickly.
The way you are doing it is slipping the clutch and is absolutely terrible for the clutch. You are now using the clutch like your brake pads/rotors and it is definitely not designed for it.
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No, I was thinking you were suggesting that you could put the manual transmission into first gear while driving 100, that is absolutely insane. The mismatch between speed from the axle and the engine is going to be too great and it will damage your transmission. Most transmissions even have protection against this so that you can't even push it in to first gear while going faster than 40 Mph unless you really force it.