Capturing the uplinks is obviously the task at hand, but you must hold onto the uplink(s) while the white outline of the circle on the HUD goes a complete 360 degrees. This will grant you one Y-Wing available for a bombing run, (one per uplink) holding BOTH uplinks will allow you two Y-Wings to lock on at a time. Providing you hold the uplinks this will repeat granting several Y-wings ready for their bombing run. If the Empire captures or "shuts down" a uplink, any progress on that particular white outline is lost and the Rebels must re-capture the uplink for the process to carry on. No Y-wings already gained/"locked on" are not lost at any point. Your number of Y-wings ready are shown in the top of the screen to the left of the line with circles. (See picture).
Once you run out of Y-wings the next uplink phase begins. Capture and hold these again for more Y-wings ready for their second bombing run. If you haven't taken both out by now, Snowspeeder's become available to the rebels in a last ditch effect to take them out with their tow cable. But this can only be done during the last Bombing run, (when their disabled). Fail to take BOTH out by the end of this stage and the empire wins by the AT-AT destroying the Shield Generator.