Chain pusher on Brompton moves upper tensioner pulley to direct chain on one or another cog. Aftermarket pushers are equipped with rollers there to reduce friction of the pusher claws against the pulley. You can get such pushers out of Ti, alu and carbon. The rollers vary between steel and POM I think.
On a stand the rollers indeed work well ensuring accurate shifting. Unfortunately, the real life situation is dynamic: the chain pushes against the pulley, the tensioner arms flex, chain jumps around. The end of the roller directed towards the pulley presents an edge that the pulley can catch. The chain let loose can push against the other edge of the pulley and escape from the space between the pulley and pusher. In fact after a while I found it impossible to end a ride without the chain escaping and taking the derailleur out of operation. I tried both original Brompton pulleys and aftermarket with the same effect. Complete disaster.
One of the photos shows the situation after a ride, with the chain having escaped the upper pulley. One other illustrates the catching of an edge of a pulley against the roller.
A Ti Pusher with Rollers
Catching an Edge
Chain Out Free