I feel the same. We know its a rhythm movement. Frequency is decreased uphill and you can apply force over a greater period. This gives you more control. There is a large transfer between short hills and starts/accel.I say work start tech and build p-chain. You can do hills quad dominantly with long contacts, but this is disaster for starts.
The foot strikes closer to the forefoot uphill. Usually runners that have flat foot contacts on horizontal ground will feel better going uphill because their foot angles are better. Additional the feel of greater efficiency comes from the fact stride length is shorter and frequency is higher.
I am not a big fan of mental cues during speed work on the track. I think 20/20/20 hard -relax- hard is a good approach to learn how to relax & sprint. Try tempo runs barefoot on grass. It will improve contact time and foot angles. I’ve found that getting to the basics is often the best approach. In the case of barefoot work, foot mechanics are reduced to their most simplest form.