The difference of Skywalker-40A and Skywalker-40A-UBEC:
Skywalker-40A (PN=80060020): Linear mode BEC, 2-3S Lipo
Skywalker-40A-UBEC (PN=80060140): Switch mode BEC, 2-4S Lipo
 
1 Specification

1.1 Output: Continuous 40A, Burst 55A up to 10 seconds.
1.2 Input Voltage: 2-3S Lipo, 5-9 cells NiMH. 
1.3 BEC: 3A / 5V Linear mode BEC.
1.4 Refresh rate of the throttle signal: 50Hz to 432Hz.
1.5 Max Speed: 210000rpm for 2 Poles BLM, 70000rpm for 6 poles BLM, 35000rpm for 12 poles BLM. (BLM = BrushLess Motor)
1.6 Size: 68mm*25mm*8mm.
1.7 Weight: 39g.

2. Features

2.1 Safety arming feature: Regardless the throttle stick position, the motor will not spin after battery connected. 
2.2 Throttle calibration funtion: Throttle range can be calibrated to be compatible with different transmitters.
2.3 User programmable. 
2.4 Multiple program methods supported: transmitter stick,LED Program Card. Very easy to program the ESC at home or at the flying field.
2.5 Compatible with Lipo and NiMH battery.
2.6 Timing can be changed to be suitable for different brushless motors.
2.7 Full protection: Low voltage cutoff protection / Over-heat protection / Throttle signal loss protection