I’m assuming you are an IMG.
Send an email to a department chairman at a big research institution. Write that you are willing to work UNPAID. Make sure you capitalize “UNPAID”.
Make sure you kiss his butt and the program directors butt so come match day they will rank you high. Use your one year wisely.
If you don’t match at said program after toiling for a year, consider it the ultimate form of betrayal/stab in the back.
Any one who is into research would love to have free labor. If you publish papers for him or her you will get a glaring letter of recommendation. Target a program where you want to do residency. Your connections with the faculty are priceless, especially those in power to make decisions.
Really you should try to get a paid position but they aren’t easy to find.
It can help you get a residency in the US if you are a IMG. Don’t underestimate the power of having connections.