Left side means you place the trade before the trend is confirmed. Yesterday, Nasdaq and QQQ are retreating to re-test its critical support at MA50, so, if you buy here, you are anticipating Nasdaq will hold MA50. The advantge of left side trade is that you can normall get a better price (because you take action before entire market does). The donwside of left side trade is that you do not really know what market will do (ie, Nasdaq could hold MA50, or go straight down from there and reverse the uptrend)
Right side means you wait for confirmation of the trend and then you place the trade. So, today, Nasdaq holds firmly above MA50 and we have much higher confidence that this retest is successful. The advantage of right side trade is that it increase your probability of being on the right trend, the downside is that you almost always end up paying "higher".
Due to this reason, when you buy 2x or 3x leveraged ETF such as TQQQ, you may want to do it as a right side trade. This is because emotionally, you are likely to lose confidence fast if you do this from the left side
