This is a very subjective term. If it's a simple barely felt nudge, then that's the torque converter engaging. If it's much stronger, there's something wrong. Send it to a reputable mechanic, posting such subjective inquiries will yield different results. I'd rather have you send it to a shop that can properly diagnose it, rather than post a solution here that may not answer your question properly.

Take this with a grain of salt. When the transmission shifts from R to D to N or P, the torque converter engages and disengages depending on the gear you set it to. You should feel it engage, but it should not be a strong jerk. How strong is strong, you ask? That's why I suggest you take it to a reputable shop that can diagnose it properly.

ATF can be drained and refilled (not a flush). Replace the pan filter as well. Check your manual for the proper fluid. Some cars use different types of ATF. High RPM will also result in stronger nudge like when cold.