Loading sharing is the ability to draw power from battery but when the battery is being charged, the load switches seamlessly to being powered from the same power input that is powering the battery charger.
Products that do this: *adafruit make a 5v boost + charger with load sharing. [1] *nitecore make a 18650 li-ion charger + usb power bank which also has load sharing. which is a rare feature for power banks. [2]
Example load share circuit: http://blog.zakkemble.co.uk/a-lithium-battery-charger-with-load-sharing/
[1] arr search youll find it, called powerboost i think. note that its not good for OTG phone use. i damaged one of mine when my device was sending power back down to the powerboost. :( [2] https://www.banggood.com/Nitecore-F1-Flexible-Micro-USB-Outdoor-Power-Bank-S... (where i bought mine from.)