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In all three competition lifts, the timing of the transition between the upward extension and drive of the lower body to the pull or push under the bar by the upper body is a critical element of optimal lift technique, as well as a point of confusion for many athletes.
 
The jerk tends to get less attention in discussions of weightlifting technical matters, but it deserves as much consideration as the snatch and clean—maybe more because of its historical lack thereof.
 
The timing of the push of the arms relative to the drive of the legs will have a significant influence on the outcome of the jerk—too early and you’ll lose bar speed and elevation and likely push the bar forward and shift the back foot and hips too far back in the receiving position; too late, and you’ll also lose bar speed and elevation.
 
The answer to the timing question can be answered in terms of bar speed or leg extension. The simple answer is that the arms should begin pushing against the bar when it reaches its maximal upward speed in order to preserve that speed as much as possible and achieve maximal height as a consequence. Of course, that begs the question of when that maximal speed is reached.

the arms should begin pushing against the bar when it reaches its maximal upward speed

Generally the entry-level advice for the jerk is to begin pushing with the arms when the legs have completed their extension—this is what I will tell beginning lifters in most cases to keep things simple. However, this needs to be refined to be perfectly accurate. The arms should begin pushing when the legs have completed the degree of extension at which they’ve produced maximal speed on the bar. When is this? It depends on the style of jerk.
 
As I’ve written about previously, I split jerk technique into two categories: strength and elastic. These categories describe what the lifter relies on primarily to accelerate and elevate the bar in the drive of the jerk: strength or elasticity. Keep in mind that both are being used in all jerks; what changes are the relative degrees of each. This is also not an either/or matter—lifters will exist on a spectrum between these two ends with varying mixes of both.
 
Briefly, a strength jerk will rely primarily on basic leg strength to drive the bar up, which will require a somewhat deeper dip, involve a slower transition time in the bottom, and then require more time in the drive up to achieve maximal bar speed. The elastic jerk will rely more on the elasticity of both the legs and the barbell, which will mean a shallower dip, a quicker transition in the bottom, and require less time in the drive up to achieve maximal bar speed.
 
This being the case, the more strength-reliant the lifter, the later in the extension of the legs the bar will reach maximal speed; accordingly, these lifters should begin pushing with the arms later in the drive, as the legs are reaching complete extension.
 
The more elastic jerker should begin pushing with the arms sooner in the drive to match the earlier timing of the barbell’s maximal speed, which will precede the complete extension of the legs. However, this does not mean that these more elastic jerkers should not complete the extension of the legs in the drive—they should follow through with this extension to continue elevating the bar (just as in the snatch or clean where maximal speed is typically achieved before complete extension). The difference will be the activity of the arms during that final portion of the extension.
 
How do you determine what you need to do? Generally you’ll gravitate naturally to the jerk style that best suits you simply because it will feel most effective to you. If you’re not sure, consider your explosiveness in general—if you’re a quick, snappy lifter with a big vertical jump using a standard countermovement, you’re probably going to be more of an elastic jerker; if instead, you’re a bit slower (but still could be very strong) and aren’t a big jumper, you’re probably going to be more of a strength jerker.
 
The best way to know is to experiment with jerks themselves. Just like in any lift, you’ll know when you do it well because you’ll feel the bar fly. More specifically, experiment with power jerks in the 70-80% intensity range—this will allow you to really feel the elasticity and height of the drive—trying varying dip speeds, depths, and timing of the arms.