Two questions: 1. If I start an activity on my watch, and leave my watch in the locker, will the HRM collect the data during my activity and sync it back to the watch once I come back to the locker and get my watch? 2. Does the strap stay in place well during vigorous activities? e.g. boxing, dance, gymnastics, where the body turns and twists a lot?
In this price segment the sensors do not store offline information as far as I know. You need something like a Wahoo Tickr X or similar if you need this function, but a sensor like that is way higher in price: around CHF 100. The strap itself is quite firm and can be adjusted but will surely loose elasticity like all other straps too. But surely made for vigorous workout as well.
In my experience, the strap stays in place well. I use if for cycling and hiking, and haven't had a problem, and you can make it quite tight. I don't think the HRM itself has any memory at all - you must wear a watch or sync it to your phone. I use an android phone with an HRV program to measure HR variability, and as long as that is within 15 meters or so, I don't have any problems.