The "bump Phil" technique has you simply knocking him off balance for a moment to allow Niko to get into the back of the cab just before Phil gets in. If you didn't bump him, he'd get in before you and then exit immediately, which ends up wasting 5-10 seconds. The reason for choosing the back left is simply because if Niko takes the back right, Phil refuses to enter for a few seconds. If you wait for Phil to regain balance, at which time he heads for the passenger door, and then force Niko to take the back right, Phil will immediately jump out of the car, which may result in him being left behind...a condition for mission failure.

Using rockets is the fastest option, but also the riskiest. Depending on the route/pace, it may actually end up costing you time.