Trendline Break & Retest is a trade where it occurs is more important for it playful significance.
Case -1
For ex: If TLBR is occuring while stock is turning up from a prolonged downtrend, then it should be played with caution, as it can be a case of breakout failure.
Tip here is observed volume during this scenario, it should be low relative to recent selling bars. Volume below this trendline consolidation has to be low, so that breakout volume can be meaningful.
Case-2
If trend is already up, and we are trading TLBR setup while having a consolidation/correction. During this scenario, chances of breakout failure is less, as we are with ongoing trend.
Tip here is, In this whole uptrend, where this consolidation occuring, is this is a first consolidation area after market turned up, or its third-fourth consolidation area. Why it matters is, if you are trading first consolidation after uptrend, chances of continuing is up, but if you playing late consolidations in uptrend, viz -4th/5th then chances of failure are higher.
Now on volume in this case is relative lower than the volume coming in recent uptrend.
Note here, volume in uptrend consolidation is always lower than volume while the stock itself is changing its trend from down to up. In short volume during consolidation in case-2 is always lesser than volume during consolidation in case-1.
------------------------------------
Now coming to your part, why it goes up/down at trendline before proceeding and what should be entry.
One can go full wither on breakout or wait to play on retest. ON retest too, you have two options either go on trendline touch, or go on the high of peak, from where this stock is correcting after a breakout.
If you had gone long on trendline retest, then SL can be your affordability wise.
But recent observations by me, i had seen stock after retesting come below trendline and spent few bars below it, without breaking low of immediate 'trough' prior to breakout. So that is safe area for conservatives to add their positions if at all it comes.
----------------------------
Every setup has failure, there cannot be 100% successful setup, but we as a trader/investor need to get into that sweet point where things dont turn much sour for us...