Quick answer
Promising God something in a hot moment, then trying to negotiate later, traps the person who did it. The trap is self-set.
Read both lines
A snare in this book is something a person walks into, not something done to him, and the word choice does the work before the explanation starts. The situation is specific to Israel's worship: declaring an animal, a field, or a sum to be holy, which put it beyond ordinary use and could not simply be undone. The second line names the sequence rather than the sin, and the sequence is the problem. Speaking first and inspecting the vow afterward reverses the order, and by then the words are already out. What the verse warns against is not devotion but haste in it, which is the same complaint the chapter made about inheritance four verses earlier.
Related: Proverbs 20:21 · Ecclesiastes 5:4–5 · Proverbs 6:1–2 · Proverbs 29:20
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.