If not already using clipless pedal/cleat systems or straps, then do something to afix your feet to the pedals so you can push/pull on both sides in order to increase energy transfer into the crank. At the same time, any maybe it is obvious, you can push your shoulder back into the seatrest in order to use the pedals as more of a stepper while climbing, despite you cannot stand up to pedal on a recumbent. Both those actions help with energy transfer into the bike, but the behind that you have to be able to sustain the additional energy output, so a bit of training as mentioned above. I am not that ambitious so I use the described method of heading in fast and shifting down periodically to maintain cadence at whatever energy level I feel like giving on a particular day.
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Longbikes Slipstream