Yes, so far as I know it's impossible to cheat in terms of the body English mentioned.
It is possible to jerk the weight at the start of the movement and thus rob some of the value if actually having the strength at that point to complete the rep smoothly and at moderate tempo, so that should be avoided.
Where desired though it can be a technique for extending a set to do as many reps as possible smoothly and at moderate tempo, generating no extra momentum in the early part of the rep, and then to get a couple more in with the aid of explosiveness. So it's not necessarily a bad thing that one is able to do that: it's just that it shouldn't, IMO, be relied on as how most of the reps are done.
Reps using explosiveness should not done, I believe, past the point where it seems that the last rep was done that could still slowly pull back the final part of the rep, hold the full contraction and lower the weight gradually.
Definitely the most benefit is in the last part of the movement and the full contraction, and so going into that with too much momentum is sort of a form of cheating.