Here's a basic explanation: beam reaches are likely to capsize you when you treat them like upwind sailing - i.e. by heading up to depower. You need to head down in the puffs.
Also, you're more likely to pitchpole (go head-over-heels). Weight ballance (all the way back) is the way to avoid this. Anticipate the puffs, and bear of immediately.
What windspeeds were you concerned about?
Good luck
Michael c.
t4.9#32