Bathroom sink pipe work leaks

[8BitJesus]

Just a quick one

If I turn the tap on to full flow, or if I put a lot of water down the plug at once, water leaks from the section I’ve highlighted in the photo

it never leaks when it’s a small flow

any ideas what it could be? I assumed that section was open to allow air in the piping and allows water to drain quickly

 

[denso13]

I assumed that section was open to allow air in the piping and allows water to drain quickly

It is, but it isn't working correctly if it is leaking. Could try cleaning it out but not expensive to replace the trap.