Although I upvoted @Jedediah 's answer as it's the most comprehensive (and correct) answer, just a few important amplifications from somebody who rides a Penny-Farthing almost daily:

Heel on Rear Wheel:
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Standing on the mount peg and pressing your heel on the rear wheel is actually a fundamental skill all Penny-Farthing riders should have. Indeed, on a Penny-Farthing without a spoon or caliper brake- _my UDC 50" MK3 Penny-Farthing is such a bike_- standing on the mount peg and pressing your heel on the rear-wheel is the only 2nd braking mechanism for the rear wheel. And this form of braking would ideally be applied at the top of the hill to moderate your speed.

I was riding my 50" Penny-Farthing with a group a few months ago and one rider actually lost a pedal going down a steep hill, so negative resistance on the pedals as a means of braking was not an option here. The rider resolved the issue by aiming for the biggest bush he could find- ***crashing***- so not ideal.  I suspect the rider probably put the pedals on backwards after transporting the bike to the meeting point, but in any event, being able to brake using your heel on the back wheel is your ace in a hole ;-).

Rear Wheel Caliper Brake:
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I also have a UDC MK4 Penny-Farthing which ships with a mechanical rear wheel caliper brake. It is an effective braking mechanism, however possibly ***too effective***: apply it too hard and the back wheel can walk out on you when it starts to bite. When using a rear wheel caliper brake use gradual, gentle pressure. But that is true of all braking I guess.

Negative Pressure on Pedals:
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If using this method, you **MUST** start at the very top of the hill.  If you either take your feet off the pedals, or the pedals are turning too quickly, you are a ***passenger*** at that point ;-).  You'd need to brake using a rear wheel brake- either a caliper, spoon or your heel once the pedals start turning too fast.

Anyhoo, just some practical considerations on braking I've observed which might keep other Penny-Farthing riders safe-